The promoter for lactate dehydrogenase A (LDHA), which catalyzes the conversion of pyruvate into lactate, has a binding site for HIF-1α (Firth et al., 1995; Semenza et al., 1996; Cui et al., 2017). LDH isoforms are comprised of all A subunits (pyruvate → lactate), all B subunits (lactate → pyruvate), or a combination of both. While LDH is present in all tissues, the ratio of LDHA to LDHB subunits is tissue specific (Adeva et al., 2013). In some cancers, isoforms with only LDHA subunits have the highest efficiency at converting pyruvate to lactate and are correlated with increased HIF-1α and VEGF expression, increased tumor size, enhanced metastatic potential, and thus a poor prognosis (Koukourakis et al., 2003; Koukourakis et al., 2005; Kolev et al., 2008). However, some researchers do not believe the LDH enzyme distribution has an impact on the ability of cells to produce or use lactate, but rather affects the rate of the equilibrium of the reaction (Bak and Schousboe, 2017). LDH has a prognostic role in solid tumors, with high LDH associated with poor progression free survival and lower disease free survival (Petrelli et al., 2015). Additionally, serum LDH has been correlated with systemic inflammatory responses in patients with advanced pancreatic cancer (Yu et al., 2017). LDH has also been detected in elevated levels in bronchoalveolar lavage (BAL) fluid of active pulmonary TB patients, and BAL LDH correlates with increased serum LDH (Emad and Rezaian, 1999). Interestingly, sputum positive cases of TB show increased serum levels of LDH1, LDH2, and LDH3, which contain four, three, or two B subunits, respectively (Sharma et al., 2010). Increases in cerebrospinal fluid lactate have been shown to correlate to increases in severity of clinical stage for tuberculous meningitis (Siddiqi et al., 2018). This indicates that LDH levels and isoform specificity and lactate levels may also be diagnostic or prognostic indicators for pulmonary TB.

Much of what is known about the role of lactate comes from extensive reviews of the tumor microenvironment (San-Millán and Brooks, 2017; Pereira-Nunes et al., 2020). Lactate accumulates in the tumor microenvironment both in hypoxic solid tumors and during aerobic conditions due to the Warburg Effect, and malignant transformation is associated with increases in glycolytic flux (Walenta and Mueller-Klieser, 2004). Lactate enhances the release of macrophage pro-angiogenic factors, increases tumor metastatic activity, enhances collagen deposition, and worsens cancer prognosis (Crowther et al., 2001; Trabold et al., 2003). Lactate is also capable of diffusing from solid tumors and malignant cells to stimulate tumor associated fibroblasts, generating extracellular matrix rearrangement which allows for tumor cell migration and increases in tumor cell mobility (Walenta and Mueller-Klieser, 2004). Interesting, cancer cells that are more oxidative are sensitive to lactate signaling whereas more glycolytic cells are not and do not take up lactate and do not respond to exogenous lactate treatment (Pérez-Escuredo et al., 2015). While cancer research has led the effort in understanding the role of lactate in modulating carcinogenesis, much learned from this work can be applied in the realm of infectious disease. Early in Mtb infection, shifts to glycolytic metabolism are observed within macrophages leading to increased lactate production (Qualls and Murray, 2015; Shi et al., 2015; Gleeson et al., 2016; Osada-Oka et al., 2019; Shi et al., 2019) and high lactate concentrations are observable within granuloma lesions, reaching levels comparable to tumors (Somashekar et al., 2011; Somashekar et al., 2012).

Lactate was largely considered to be a metabolic waste product of glycolysis until the 1980s, when the shuttling of lactate between cells and tissues was first described (Gladden, 2004; Brooks, 2009; Sun et al., 2017; Brooks, 2018; Ferguson et al., 2018; Brooks, 2020). One of the first described lactate shuttles was within muscle, with fast-twitch, glycolytic muscle fibers accumulating and releasing lactate and slow-twitch, oxidative muscle fibers importing and metabolizing lactate for energy (Brooks, 1985; Brooks, 1986; Brooks et al., 1999). Mitochondria have a role to play in shuttling lactate intracellularly, with LDH enzymes present in the mitochondrial intermembrane space and in the mitochondrial matrix, and monocarboxylate transporters (MCTs) present in the inner mitochondrial membrane to transfer lactate (Brooks et al., 1999; De Bari et al., 2004; Passarella et al., 2008). This is further evidenced by LDHB localizing to the mitochondria at greater densities than to other locations, and LDHA not localizing to the mitochondria (Chen et al., 2016). Additionally, mitochondrial LDH is highly expressed and more active in cancerous prostate cells than normal ones, contributing to the anaplerosis of TCA cycle intermediates to help fuel cancer progression (De Bari et al., 2010). However, some have demonstrated that lactate must be first converted to pyruvate before it can enter the mitochondrial matrix and have challenged the hypothesis that lactate can be directly oxidized by mitochondria (Jacobs et al., 2013; Passarella et al., 2014). Lactate shuttles have also been described between astrocytes and neurons, and mitochondrial LDH has been shown to play a role in allowing astrocytes to use lactate in the production of ATP via oxidative phosphorylation (Lemire et al., 2008; Magistretti and Allaman, 2018). Lactate has been demonstrated in multiple studies to be effectively metabolized by cancer cells and, in some instances, lactate is preferentially metabolized over other energy sources such as glucose (Kennedy et al., 2013; Faubert et al., 2017; Hui et al., 2017).

Monocarboxylate transporters (MCTs) are responsible for shutting lactate across membranes. Multiple MCTs exist, and cellular expression of MCTs depends on the physiological role of the cell (Halestrap, 2012; Felmlee et al., 2020). Highly glycolytic cells express MCT4, a low affinity transporter adapted to the export of lactate, while more oxidative cells predominantly express MCT1, which has a higher substrate affinity and is adapted for lactate import (Dimmer et al., 2000; Kirk et al., 2000). MCT1 has been demonstrated to be expressed in macrophages and increased upon LPS, TNF-α, or NO treatment (Hahn et al., 2000). The expression of MCT1 and MCT4 is associated with the expression of the chaperone CD147, which has transmembrane and cytoplasmic domains that interact with MCTs and help organize their distribution and localization on the plasma membrane (Kirk et al., 2000; Walters et al., 2013). CD147 has pro-tumor effects via its control of lactic acid transport, and knocking down CD147 reduces MCT1 and MCT4 expression and reduces the glycolytic rate by 50% (Le Floch et al., 2011). The invasive capability of human lung cancer cells was correlated with the expression of MCT1 and MCT4 and proliferation was reduced when these MCTs were inhibited (Izumi et al., 2011). Importantly, MCT4 is hypoxia-inducible, exhibiting 3-5 fold mRNA increases during hypoxia and showing significantly higher tissue expression in hypoxic areas as shown by co-localization with pimonidazole staining (Ullah et al., 2006; Le Floch et al., 2011; Rademakers et al., 2011). CD147 expression has also been demonstrated to be regulated by HIF-1α, promoting glycolysis and inhibiting tumor cell apoptosis (Ke et al., 2012; Ke et al., 2014). In some cancers, MCT1, MCT4, and CD147 have been identified as prognostic biomarkers which predicts poor patient survival (Latif et al., 2017; Luz MC de et al., 2017; Ruan et al., 2017). Within the context of Mtb infection, less is known about the presence and function of MCT1, MCT4, and CD147. CD147 has been demonstrated to be a marker of activated, highly suppressive regulatory T cells, and in vitro Mtb antigen stimulation expanded CD147 positive regulatory T cell subsets (Feruglio et al., 2015). Evaluation of mRNA from Mtb infected mouse lungs showed an increase in mRNA copy number of MCT4 over the course of infection, while MCT1 levels remained unchanged, indicating that high lactate efflux is an important host response to Mtb infection (Shi et al., 2015). Increased macrophage MCT4 was also confirmed recently via a transcriptional analysis of Mtb infected macrophages (Vrieling et al., 2020). MCT4 is also upregulated in macrophages upon stimulation of TLR2 and TLR4, through which Mtb antigens also stimulate macrophages responses, and knock-out of MCT4 can reduce macrophage proinflammatory cytokine production (Tan et al., 2015). MCT4 has also been shown to be present within the phagosomal membrane of Mycobacterium bovis BCG infected macrophages (Lee et al., 2010). Despite this, analysis of the dynamic role of MCTs in transporting lactate within the TB granuloma has not been described.

Combining knowledge about LDHs and MCTs, the lactate shuttle hypothesis posits that a metabolic symbiosis between hypoxic and oxidative tumor cells exists, wherein hypoxic, glycolytic cells produce lactate at high levels, and this lactate is transported to oxidative tumor cells, which convert lactate back into pyruvate to fuel their metabolism, and conserve glucose for hypoxic cells (Sonveaux et al., 2008) ( Figure 2 ). Metabolically distinct cell populations should have unique signatures showing different expression levels of relevant metabolic enzymes and transporters, with glycolytic cells having increases in MCT4, LDHA, HIF-1α, while oxidative cells will have MCT1, LDHB, and will accumulate lactate (Nakajima and Van Houten, 2013). This hypothesis takes into account that solid tumors can be avascular, relying on diffusion and blood flow to obtain oxygen and glucose (Kim et al., 2011; Fadaka et al., 2017). Hypoxia, however, can initiate angiogenesis, creating a new capillary network to the tumor (Kim et al., 2011; Fadaka et al., 2017). Interestingly, models demonstrate that tissues have easier access to glucose within tumors with more chaotic microcirculation, and cells with high glycolytic capacity are more efficient at taking up limited glucose supplies, have higher lactate outputs, and maintain more phosphorylated glycolytic intermediates than cells with lower glycolytic capacity (Østergaard et al., 2013; van Beek, 2018). Glucose also appears to diffuse more readily in malignant tumors, such as adenocarcinoma and squamous cell carcinoma, than in non-malignant tissue (Zhao et al., 2010). This all supports the notion that peripheral cells in more oxidative environments preserve glucose for highly glycolytic, hypoxic cells. Metabolic modulations which characterize the TB granuloma are similar to those described within the tumor microenvironment. Therefore, these enzyme and transporter expression profiles, which occur in different regions of the tumor microenvironment, may directly apply to different regions of the granuloma microenvironment.

Indeed, a lactate shuttle has been proposed between astrocytes and microglia within the context of tuberculous meningitis (TBM), based on proton magnetic resonance based metabolomics of cerebrospinal fluid (CSF) samples of TBM-confirmed children (Mason et al., 2015). It is hypothesized that astrocytes respond to Mtb infected microglia by mobilizing glucose, increasing extracellular lactate, and increasing CSF lactate; this lactate is then used by microglia to fuel oxidative phosphorylation and reactive oxygen species production (Mason et al., 2015; Mason, 2017). In the presence of activating stimuli, such as LPS and IFNγ, microglia also convert glucose to lactate (Gimeno-Bayón et al., 2014). The role of lactate in the host response to TBM has been further highlighted, as high CSF lactate is a hallmark of TBM and all CSF lactate from TBM cases was found to be of the L-lactate form, indicating it was produced by the host rather than Mtb (Mason et al., 2016). Like macrophages present within pulmonary granulomas during TB disease, persistently activated microglia exhibit both neuroprotective and neurodestructive properties, contributing to nuances of host-pathogen dynamics in TBM (Hall et al., 1998; Giaume et al., 2007). Thus, further exploration of lactate shuttle dynamics and an evaluation of the expression of involved enzymes and transporters within the lung granuloma microenvironment is warranted.

Many cell types interact to shuttle and utilize lactate. Lactate modulates endothelial cells, and MCT1 driven lactate uptake in endothelial cells leads to significant increases in IL-8 production through the NFκβ-pathway (Végran et al., 2011). Additionally, lactate released from cancer cells via MCT4 stimulates IL-8 production, tumor angiogenesis, and tissue perfusion, supporting a model where tumor cell lactate can stimulate angiogenesis in endothelial cells (Végran et al., 2011). The impact of lactate on angiogenesis has been shown to be critical for the acceleration of healing in ischemic wounds, preventing muscle atrophy, and stimulating the production and release of angiogenic factors such as IL-8, bFGF, VEGF, and VEGFR (Xiong et al., 1998; Porporato et al., 2012). Crosstalk between cancer cells and endothelial cells via lactate release and MCT transport of lactate has also been demonstrated in glioma models (Miranda-Gonçalves et al., 2017).

In what has been deemed the “Reverse Warburg Effect,” cancer cells secrete hydrogen peroxide, which induces oxidative stress in cancer associated fibroblasts, causing them to undergo aerobic glycolysis in response, which produces large amounts of lactate that is then shuttled to tumor cells to fuel mitochondrial oxidative phosphorylation (Whitaker-Menezes et al., 2011; Fu et al., 2017; Koukourakis et al., 2017; Wilde et al., 2017). Further evidence of this is that MCT4, which exports lactate, is present in cancer associated fibroblasts and is up regulated by oxidative stress, and cancer cell MCT1 is up regulated when co-cultured with fibroblasts (Whitaker-Menezes et al., 2011; Fu et al., 2017; Koukourakis et al., 2017; Wilde et al., 2017). This metabolic synergy between cancer associated fibroblasts and cancer cells allows tumors that have been highly infiltrated by stromal cells to shift stromal metabolism to produce large quantities of lactate and export it via MCT4, and allow tumor cells to exploit the lactate produced, via MCT1 influx, and use it to fuel metabolism under glucose limited environments (Fiaschi et al., 2012; Curry et al., 2013). Lactate stimulates MCT1 expression in stromal cells, but reduces it in tumor cells, further suggesting that lactate is exported by tumor cells and taken up by stromal cells (Rattigan et al., 2012). Thus, the metabolic perturbations within the tumor microenvironment create spatial gradients based on differential sensitivity to lactate between cancer cells and stromal cells, such as tumor associated macrophages, and restricted perfusion contributes to these metabolic gradients, with cancer cells able to survive better in high lactate, hypoxic regions (Carmona-Fontaine et al., 2013). Spatial gradients also exist within the context of the TB granuloma, with different immune transcripts and proteomic profiles within necrotic, central regions as compared to the rim of the granuloma (Marakalala et al., 2016; Carow et al., 2019). Oxygen tension gradients and altered vasculature associated with granulomas also lead to restricted perfusion and central regions of hypoxia (Via et al., 2008; Datta et al., 2015; Datta et al., 2016). Based on these similarities between the granuloma and tumor microenvironments, it is likely that spatial differences in lactate metabolism and diverse crosstalk between heterogeneous cell types exists within the context of the TB granuloma. However, how differences in lactate metabolism and signaling manifest between different granuloma types, and their variable architecture and predominant immunologic cell type, is a remaining knowledge gap.

Importantly, the lactate shuttle has been explored as a therapeutic target for diseases such as cancer, as lactate is a key regulator of carcinogenesis (Danhier et al., 2013; Doherty and Cleveland, 2013; Jones and Morris, 2016; Marchiq and Pouysségur, 2016; San-Millán and Brooks, 2017; Pivovarova and MacGregor, 2018). To exploit the need for glycolytic and oxidative tumor cells to shuttle lactate, researchers have explored MCT1 inhibition with α-CHC and have demonstrated that MCT1 inhibition can slow tumor progression and growth (Sonveaux et al., 2008). Lactate, released in the hypoxic tumor cell compartment, fuels the oxidative metabolism of cells that are in more vascularized, oxidized regions. This spares glucose for use in the hypoxic cells, which depend heavily on glycolysis. Inhibiting MCT1 thus induces a switch in oxidative cells from lactate fueled respiration to glycolysis; hypoxic cells die from glucose starvation, and cells that remain are more oxygenated and more susceptible to chemotherapies and radiation (Sonveaux et al., 2008; Mendoza-Juez et al., 2012). In malignant glioma, silencing MCTs decreases lactate efflux, and induces apoptosis and necrosis (Mathupala et al., 2004). Activated T cells rely on glycolysis for energy, produce increased amounts of lactate, and have higher MCT1 and MCT4 expression than resting cells; as a result, blocking lactate transport via MCT1 inhibits rapid phases of T cell division (Murray et al., 2005). Targeting MCT1 in endothelial cells can block the activation of HIF-1α by preventing lactate uptake and its conversion to pyruvate by LDHB, which subsequently competes with 2-oxoglutarate to inhibit PHDs needed to degrade HIF-1α (Sonveaux et al., 2012). This also blocks the downstream activation of angiogenesis. Inhibiting MCT1 can impair glycolysis and upregulate mitochondrial metabolism, improving the cellular bioenergetic state (Beloueche-Babari et al., 2017). Additionally, disrupting CD147, and therefore MCT1 and MCT4 expression, can sensitize lung cancer cells to biguanides like phenformin and metformin (Granja et al., 2014). Sensitization to phenformin could also be achieved by genetic knock out of MCT1 and CD147 (Marchiq et al., 2015). MCT1 knockdown is more effective under hypoxic conditions, resulting in larger decreases in lactate levels, cell biomass, cell invasion, and drastic reduction in breast cancer growth, and treatment with metformin could increase the response/efficacy of the MCT1 inhibition (Morais-Santos et al., 2015). The research combining MCT inhibition and biguanides is of interest for TB therapy, as metformin has been actively explored as a HDT (Lin et al., 2018; Lachmandas et al., 2019; Oglesby et al., 2019; Yew et al., 2019; Abinaya et al., 2020; Frenkel et al., 2020; Rodriguez-Carlos et al., 2020). However, inhibiting MCTs has yet to be explored in the context of Mtb infection. Based on the aforementioned accounts of increased MCT expression during Mtb infection, it is a viable area for future research.

Targeting LDHA has also been investigated as a way to increase the aerobic metabolism of pyruvate within the mitochondria, increasing reactive oxygen species production, oxidative damage, and cancer cell apoptosis (Doherty and Cleveland, 2013; Miao et al., 2013; Zhao et al., 2013; Gray et al., 2014). LDHA inhibition was found to increase apoptosis of A549 cancer cells (Xie et al., 2009). Inhibiting LDHA with compound FX11 decreased ATP levels, reduced mitochondrial membrane potential, increased oxidative stress linked to cell death, activated AMPK, and inhibited tumor xenograft progression in transformed human B cell lymphoma (Le et al., 2010). Simultaneously inhibiting LDHA via sodium oxamate and respiratory complex I via metformin depletes the cellular ATP pool, causing cancer cells to undergo metabolic catastrophe, leading to tumor cell growth arrest and cell death, leading to significant decreases in tumor size (Chaube et al., 2015). LDH inhibition via oxamate can also induce autophagy through inhibition of the AKT/mTOR pathway, reducing phosphorylation of Akt and downstream mediators mTOR and p70s6k (Zhao Z et al., 2015). Interestingly, if autophagy is inhibited, oxamate induces apoptosis and inhibits proliferation of cancer cells, indicating that the combination of oxamate with other inhibitors, such as those that prevent protective autophagy in cancers, are viable approaches (Zhao Z et al., 2015). Recent work demonstrates that inhibiting LDH via FX11 reduces the bacterial burden in Mtb infected mouse lungs, reduces the number of necrotic lesions, and potentiates isoniazid treatment (Krishnamoorthy et al., 2020). This may be due, in part, to the fact that Mtb is able to oxidize lactate via a quinone-dependent L-lactate dehydrogenase and, interestingly, lactate oxidation appears to be required for intracellular Mtb growth (Billig et al., 2017). Inhibiting macrophage production of lactate may therefore be a viable host-directed therapeutic strategy for combating Mtb infection. It has also been demonstrated that production of lactate through HIF-1α induced LDHA expression was critical for preventing intracellular pyruvate accumulation in Mtb infected macrophages (Osada-Oka et al., 2019). Interestingly, it was found that Mtb more efficiently used pyruvate over glucose to fuel its intracellular growth (Osada-Oka et al., 2019). LDHA may be required for the host to effectively restrict Mtb pyruvate use. Therefore, there may be dual roles for macrophage lactate production within the context of Mtb infection, as a result of a dynamic interaction between host and pathogen. This poses a unique challenge to the TB field that is not present in cancer. Additionally, the efficacy of these therapeutic strategies may be impacted by the type of granuloma, the stage of disease progression, and the host disease model. It is also important to consider the effects of modulating highly conserved metabolic pathways, such as lactate metabolism, on overall host health. For example, LDHA deficiencies have been shown to cause pustular psoriasis in some patients (Takeo et al., 2016). The relative impact of a therapeutic intervention on host cells must be accounted for when pursuing HDT for TB. Overall, a better understanding of metabolic fuel use between host and pathogen is critical for pursuing lactate shuttle inhibition as a therapeutic strategy for TB.

Lactate also has distinct immunomodulatory effects (Morrot et al., 2018). Accumulated lactate in the tumor microenvironment, which can reach up to 40 mM concentrations, decelerates energy metabolism and induces suppressed phenotypes in infiltrating T cells by decreasing antigen-specific proliferation, and impairing T cell production of IL-1, IFN-γ, perforin and granzyme B (Fischer et al., 2007; Choi et al., 2013). Lactate inhibits CD4+ T cell chemotaxis, induces IL-17 production, and inhibits CD8+ T cell cytolytic activity (Haas et al., 2015). Macrophage LDHA is critical for macrophage dependent activation of antitumor CD8+ T cells and boosts IL-17 mediated immunity (Seth et al., 2017). IFN-γ producing CD4 T cells are predominant responders in anti-mycobacterial immunity; however, their role in TB pathogenesis is complex and these cells can stimulate neutrophil and macrophage mediated disruption of granuloma tissue architecture (Kumar, 2017). Cytolytic CD8 T cell function plays a critical role in Mtb infection, especially for long term control of infection (Lin and Flynn, 2015). Th-17 cells also play a diverse role in the pathogenesis of Mtb infection, particularly through their recruitment of neutrophils to the site of infection (Lyadova and Panteleev, 2015; Shen and Chen, 2018). Accounts of the relationship between T cells phenotype and lactate within TB are limited; however, the levels of lactate within the TB granuloma reach mM concentrations comparable to that of tumors, and LDHA expression colocalizes with CD3+ T cells in mouse lungs 30 days post infection (Somashekar et al., 2011; Shi et al., 2015). Based on this, it is likely that lactate may exert some of these described effects during Mtb infection. The level of lactate within the microenvironment may in part shape the type of granuloma lesion that develops, impacting therapeutic strategies. Specifically, the induction of IL-17 production and boost of IL-17 mediated immunity and simultaneous reduction in CD4+ and CD8+ T cell proliferation, infiltration, cytokine production, and effector activity via lactate may lead to described type II lesions, which are rapidly progressive and composed almost entirely of neutrophils (Irwin et al., 2015). Understanding how lactate concentrations vary within different TB granuloma types may shed insight into their immunologic phenotype and character.

Regulatory T cells (Tregs) are resistant to the suppressive effects of lactate on effector T cells, making these populations better able to survive in high lactate environments and assist in cancer immune evasion (Angelin et al., 2017). It has been shown that TB patients who have negative sputum smears post treatment have significantly lower levels of plasma IL-17, higher levels of plasma IL-10, and higher percentages of Tregs when compared to sputum positive individuals, potentially pointing to a role of Tregs in controlling TB progression (Xu et al., 2016). However, Tregs can antagonize the protective cellular immunity of Th1 and Th17 responses, which could result in loss of control of TB disease and the production of anti-inflammatory mediators by Tregs early in infection could enhance mycobacterial growth by suppressing the immune response (Brighenti and Ordway, 2017). In contrast, Tregs are also required to control inflammation, especially in prevention of autoimmune disorders. Interestingly, the regulation of the Th-17 versus Treg balance has been demonstrated to be a result of HIF-1α regulation, with active HIF-1α contributing to the differentiation of Th17 cells and a lack of HIF-1α enhancing Treg development (Dang et al., 2011; Shi et al., 2011). The timing of HIF-1α activation and its subsequent impact on increased lactate production may be key to determining whether an inflammatory or anti-inflammatory granuloma microenvironment forms, and impact the course of Mtb infection. A delicate balance of T cell regulation is key to the enhancement or resolution of TB pathology, and lactate accumulated within the granuloma may play a critical role in augmenting this balance.

NK cell production of granzyme B and perforin is also reduced in the presence of lactate, and NK cells treated with lactate express fewer activation receptors (Husain et al., 2013). Additionally, LDHA deficient tumors have significantly higher NK cell activity and fewer myeloid derived suppressor cells present (Husain et al., 2013). Tumor derived LDH can induce NK cell ligands on myeloid cells, which causes NK cells to down regulate associated receptors and impairs their anti-tumor activity (Crane et al., 2014). The high levels of lactic acid that inhibit CD8 T cell and NK cell activity is mainly through the inhibition of the transcription factor NFAT (Brand et al., 2016). NK cells are known to play a role in Mtb infection, via cytotoxic release of perforin and granzyme as well as through activation of macrophages and enhancing phagolysosomal fusion via IL-22 and IFN-γ production (Liu et al., 2017). Impaired NK cell activity via lactate-mediated mechanisms could reduce the phagolysosomal fusion capacity of macrophages, aiding the ability of Mtb to set up its intracellular residence in phagosomes. Decreased functionality of NK cells may prevent appropriate immune responses necessary to activate macrophages and induce mycobacterial killing, providing Mtb an additional advantage. Additional research is needed to explore the role of lactate in augmenting NK cell immunity within the context of TB.

Tumor derived lactate also significantly inhibits monocyte TNF-α secretion, impairs monocyte glycolytic flux, stimulates macrophage VEGF and TGFβ, up regulates monocyte IL-23 production, and inhibits the differentiation of monocytes to dendritic cells (Dietl et al., 2010; Goetze et al., 2011). Migration of monocytes is inhibited, while the migration of cancer cells is enhanced in lactate enriched environments, further contributing to tumor immune escape (Goetze et al., 2011). In the presence of lactate, GM-CSF activated macrophages are not able to produce pro-inflammatory cytokines and lactate facilitates tumor associated macrophages to undergo M2 polarization, even diverting differentiation of monocytes from dendritic cells toward M2 macrophages (Su et al., 2014; Zhao Y et al., 2015; Selleri et al., 2016; Ohashi et al., 2017). This would reduce antigen presentation and induce tissue remodeling. Similarly, Mtb is known to escape the immune system by reducing antigen presentation, and impairing dendritic cell maturation and migration, as well as by inhibiting phagolysosomal fusion and inhibiting apoptosis (Grace and Ernst, 2016; Ernst, 2018; Zhai et al., 2019). Lactic acid can also interfere with TLR signaling and delay the chemokine and cytokine response to LPS stimulated inflammation (Peter et al., 2015). Deleting LDHA in specific tumors reverses immunosuppression in the tumor microenvironment, likely through reduced PD-L1 and VEGF (Seth et al., 2017). The anti-inflammatory nature of lactate has also been demonstrated in models of intestinal inflammation, whereby treatment with lactate prevented TNBS induced colitis, reduced epithelial damage and edema, decreased the production of circulating IL-6, and prevented bacterial translocation to the liver (Iraporda et al., 2016). The levels of lactate within the TB granuloma have been shown to increase over the course of infection (Somashekar et al., 2011). An increase in M2 macrophage polarization, reduced antigen presentation, and impaired cytokine and chemokine responses as a result of high lactate concentrations may contribute to a shift in host responses during late stages of infection that promotes Mtb persistence and reduces pro-inflammatory macrophage responses. Lactate likely plays a critical role in regulating monocytes, macrophages, and dendritic cells critical for responses to Mtb infection and antigen presentation and, the amount of lactate present may influence the manifestation and progression of TB disease.

Ultimately, both the tumor microenvironment and the TB granuloma are formed and sustained by complex interactions between T cells, macrophages, and other immune cells. Both disease processes rely on immunosuppressive macrophage populations to prevent immune system elimination and appropriate infiltration of T cells and antigen presenting cells to elicit antigen-specific immune responses, as has been previously reviewed (Bickett and Karam, 2020). Thus, findings on the immunoregulatory role of lactate within the tumor microenvironment have merit to be translated to the context of TB disease. However, there are some differences between these disease processes that should be noted. First and foremost, TB disease is caused by an infectious agent, and an additional layer of complexity is added when the ways in which Mtb itself influences immune cell function are considered. Tumor cells are rapidly proliferating, whereas macrophages which harbor Mtb are not, and Mtb undergoes phases of slowed growth and persistence which contribute in part to latency of infection (Voskuil et al., 2003; Garhyan et al., 2015; Getahun et al., 2015; Hudock et al., 2017; Certo et al., 2020). Mechanistically speaking, lactate in the context of chronic inflammation has been shown to actively contribute to the retention of inflammatory T cells within tissues, and have different patterns of mitochondrial enzyme localization than in cancer (Pucino et al., 2019). Lactate can trigger intracellular signals that promote chronic inflammatory processes in inflammatory disease contexts, and has the potential to do this independently of HIF-1α (Lee et al., 2015; Pucino et al., 2019). CD4+ T cells in inflammatory environments that sense high lactate levels through a transporter known as SLC5A12 are pushed to differentiate into inflammatory subsets (Haas et al., 2015). Interestingly, there are some accounts which indicate that lactate results in diminished IFN-γ within the tumor microenvironment, but stimulates IFN-γ production in inflammatory environments (Brand et al., 2016; Peng et al., 2016; Certo et al., 2020). This would have significant implications for anti-Mtb immune responses. Variable granuloma types are also composed of different immunologic cell types at varied levels of activity, which impacts Mtb survival and severity of pathology (Irwin et al., 2015). The role of the immune system and goal of therapy is also different, in that for cancer, an abnormal population of human cells are being targeted, distinct from immune cells modulating the environment, whereas to combat Mtb, macrophages harboring Mtb themselves must be activated to enhance mycobacterial killing. Mode of cell death has an impact on Mtb disease, and thus selective killing of infected macrophages may not be a viable strategy, as is the case for selective targeting of tumor cells (O’Sullivan et al., 2007; Abebe et al., 2011; Lee et al., 2011; Srinivasan et al., 2014; Certo et al., 2020).

The differences and similarities between cancer and TB must be weighed accordingly when pursuing research questions regarding the role of lactate, and the potential for modulation of lactate metabolism as a therapeutic strategy, in either context ( Table 1 ). However, given the limited number of mechanistic studies related to the role of lactate during Mtb infection and TB disease progression, the similarities pose interesting questions for further exploration. Overall, what is clear from the previous discussion is that lactate’s role in modulating immune cell bioenergetics and cellular phenotype points to more than just a role as an intermediary in metabolism, but points to critical roles in cell-cell signaling.

Lactate is one of many metabolites which signals through G-protein coupled receptors (Offermanns, 2014; Husted et al., 2017; Offermanns, 2017; Ristic et al., 2017). Lactate was revealed to be an endogenous ligand for GPR81 through the discovery that GPR81 activation in adipose tissue suppressed lipolysis and mediated insulin-induced antilipolytic effects in an autocrine fashion (Lee et al., 2001; Cai et al., 2008; Liu et al., 2009; Ahmed et al., 2010). GPR81 is expressed in brown fat, white fat, kidney, liver, skeletal muscle, brain, and lung, among other tissues and expression profiles are similar across mouse and human tissue (Liu et al., 2009). Receptor structural residues are also conserved across species from humans to zebrafish (Kuei et al., 2011). GPR81 is coupled to a Gi subunit, which inhibits adenylyl cyclase, decreasing intracellular cAMP and causing protein kinase A (PKA) to be less active (Cai et al., 2008; Jeninga et al., 2009). Downstream signaling of GPR81 also activates the ERK1/2 pathway, PKC, PI3K, and Src kinases (Li et al., 2014). As previously mentioned, PKA has the ability to activate HIF-1 in a hypoxia-independent manner (Bullen et al., 2016; McNamee et al., 2016). Interestingly, since elevated cAMP and PKA signaling inhibits IFN-γ secretion by T cells during Mtb infection, activation of GPR81 may be a viable strategy to enhance Mtb directed immunity (Chung et al., 2008). Thiazolidinediones such as rosiglitazone can induce the expression of GPR81 through PPARγ, as a conserved PPARγ:retinoid X receptor-binding site is present in the promoter of the Gpr81 gene (Jeninga et al., 2009). PPARγ has been demonstrated to have a role in Mtb infection, as knock-out of PPARγ in lung macrophages reduces Mtb growth and reduces granulomatous inflammation (Guirado et al., 2018). Mtb also limits host cell apoptosis through PPARγ induction (Arnett et al., 2018). Based on these observations, activation of PPARγ and subsequent induction of GPR81 expression may actually benefit Mtb by protecting its survival niche. Lactate signaling through GPR81, if present during Mtb infection, may have dual roles during infection, and future research to explore these hypotheses is warranted. Multiple synthetic agonists of GPR81 have been identified, some of which are commercially available, including 3-chloro-5-hydroxybenzoic acid and 3,5 dihydroxybenzoic acid (DHBA) (Dvorak et al., 2012; Liu et al., 2012; Sakurai et al., 2014; Geyer et al., 2015; Davidsson et al., 2020). These agonists have been investigated as alternative dyslipidemic agents to niacin, which signals through GPR109A and causes an undesired flushing reaction due to prostaglandin activation of Langerhans cells, but have yet to be actively explored as therapeutics for infectious disease applications (Dvorak et al., 2012; Liu et al., 2012; Sakurai et al., 2014).

GPR81 plays a role in signal transduction across multiple body systems. Inflammation in adipose tissue initiated by LPS, zymosan, or turpentine stimuli resulted in the decreased expression of GPR81 (Feingold et al., 2011). GPR81 is also down regulated when adipocytes are exposed to macrophage derived inflammatory mediators via conditioned media (Rooney and Trayhurn, 2011). Further, lactate suppresses the induction of the NLRP3 inflammasome and IL1β via GPR81 activation, regulates TLR activation via interactions with arrestin B2 downstream of GPR81, and protects against inflammatory injury in pancreatitis and hepatitis models through GPR81 signaling (Hoque et al., 2014). Mtb infection has been demonstrated to inhibit NLRP3 inflammasome activation and IL-1β processing, and increased inflammasome activity has been associated with protection against active TB (Master et al., 2008; Briken et al., 2013; Souza de Lima et al., 2016). GPR81 has been demonstrated to be expressed by non-hematopoietic and immune cells such as macrophages and neutrophils, and these innate immune cells expressing GPR81 were critical in protecting mice from experimental colitis, and were involved in regulating the balance between IL-17 producing cells and Tregs (Ranganathan et al., 2018).

Within the CNS, GPR81 is concentrated along the plasma membranes of vascular endothelial cells, acting as a volume transmitter and serving as a feedback mechanism to counteract damage and save energy under hypoxic conditions (Bozzo et al., 2013; Lauritzen et al., 2014; Morland et al., 2015; Mosienko et al., 2015; Abrantes H de et al., 2019). GPR81 is highly enriched in leptomeningeal fibroblast like cells surrounding pial blood cells, and its activation stimulates cerebral VEGF and angiogenesis (Morland et al., 2017). However, in some conditions, such as ischemic brain injury, inhibiting lactate signaling through GPR81 can reduce ischemia induced apoptosis and provide neuroprotection, potentially indicating a dual role of lactate at low versus high concentrations (Shen et al., 2015). Importantly, within the context of the TB granuloma, vascularization and angiogenesis are important for enhancing oxygen delivery and drug delivery to central, hypoxic lesions. Angiogenesis has also been implicated in allowing for increased dissemination of Mtb, as inhibiting angiogenesis via VEGF inactivation abolishes mycobacterial spread (Polena et al., 2016). Thus, lactate’s role in endothelial cell signaling through GPR81 may augment TB pathogenesis.

GPR81 has been shown to play a critical role in cancer, where silencing GPR81 in tumor cells reduced tumor growth, metastasis, cell proliferation, and mitochondrial activity (Roland et al., 2014; Brown and Ganapathy, 2019). GPR81 expression is significantly higher in cancer tissue compared to adjacent noncancerous tissues and GPR81 promotes cancer aggressiveness, malignant phenotype, cell migration, and promotes amphiregulin transcription, contributing to angiogenesis (Lee et al., 2016). Tumor cell derived lactate can activate GPR81 on dendritic cells, and prevent the presentation of tumor specific antigens to other cells (Brown et al., 2020). Further, GPR81 signaling facilitates higher rates of fatty acid synthesis by inhibiting lipolysis, which allows for cancer cells to have increased viability and higher proliferation rates (Stäubert et al., 2015). Mtb is known to rely on fatty acids and augment host lipid metabolism during infection (Singh et al., 2012; Lee et al., 2013). Signaling through GPR81 has been shown to induce the expression of PD-L1, which can lead to suppressed T cell function and immune evasion in the tumor microenvironment, and blocking lactate production can improve PD-1 targeted immune checkpoint therapies (Feng et al., 2017; Daneshmandi et al., 2019; Feichtinger and Lang, 2019). PD-1 has been demonstrated to interfere with effector T cell function during Mtb infection, and blocking PD-1 can bolster IFN-y responses (Jurado et al., 2008). However, mice deficient in PD-1 have been shown to be very susceptible to Mtb infection (Lázár-Molnár et al., 2010; Tousif et al., 2011). Human clinical case reports also suggest that PD-1 blockade can increase the severity of TB (Barber et al., 2019). GPR81 activation stimulates the expression of ABC transporters which contribute to chemoresistance of cancer cells and GPR81 signaling enhances DNA repair mechanisms (Wagner et al., 2017a; Wagner et al., 2017b). These transporters may play a role in cellular resistance to antimicrobial therapies for Mtb. Additionally, GPR81 agonism can induce a vasoconstriction response, diverting blood flow away from the kidney and inducing hypertension across multiple species (Wallenius et al., 2017). Vasoconstrictive responses may impede blood flow to the TB granuloma, contributing to tissue hypoxia.

GPR81 plays a critical role in many processes that are important to the pathogenesis of TB. The anti-inflammatory responses stimulated by lactate activation of GPR81 may contribute to immune evasion by Mtb but may also be critical for controlling chronic inflammatory processes and limiting disease severity. The evaluation of the role of this receptor during Mtb infection has yet to be conducted, either via agonizing or antagonizing this receptor in vitro and in vivo, but GPR81 shows promise as a novel HDT target to modulate TB disease.