Macrophages play a crucial role in the immune response of patients with PCOS who develop IR. They serve as primary actors in innate immunity and also function as antigen-presenting cells in specific immunity. While not all patients with an IR phenotype show signs of obesity, it is evident that obesity significantly increases the risk of metabolic syndrome in individuals with PCOS (10). In obese PCOS patients with decreased levels of lipocalin (11), there is a substantial increase in both the size and quantity of adipocytes within a short timeframe. This rapid growth leads to local tissue ischemia and hypoxia, triggering the accumulation of macrophages, which can expand to more than five times their original size (12–14). Activated and differentiated M1 macrophages are predominantly located in a ring-like structure surrounding dead adipocytes, known as the crown-like structure (CLS), where they phagocytose these cells and sustain the release of pro-inflammatory cytokines (15).

Clinical studies have shown increased expression and phosphorylation levels of Forkhead box O1 (FOXO1) in peripheral blood macrophages from patients with PCOS (16). Insulin signaling in macrophages leads to the inactivation and nuclear exclusion of FOXO1 during its phosphorylation, a process that can be reversed under conditions of IR or inflammation (17). Silencing FOXO1 in macrophages results in monocytes exhibiting a greater polarization towards the M2-type, which helps to reduce the inflammatory state and IR (18, 19). On the other hand, upregulating FOXO1 promotes the release of Toll-like receptor 4 (TLR4)-mediated pro-inflammatory factors such as IL-1β, IL-6, and TNF-α (16).

TNF-α is recognized as the initial chronic inflammatory cytokine discovered in adipose tissue linked to obesity. TNF-α has the ability to diminish insulin expression by reducing the levels of insulin receptor substrate 1 (IRS-1) and glucose transporter 4 (GLUT4) proteins. Additionally, TNF-α can trigger inflammatory pathways like nuclear factor kappa-B (NF-κB) and c-Jun N-terminal kinase (JNK) in adipocytes. This activation hampers insulin sensitivity further under the influence of inflammatory factors, thereby worsening IR (20–22).

Obesity, inflammation, and IR show a mutually reinforcing relationship, i.e., increased accumulation of adipocytes promotes macrophage aggregation and differentiation, thereby inducing inflammation and stimulating IR.

In lean patients with PCOS, abnormalities in adipose tissue persist (11). In the subcutaneous adipose tissue of PCOS patients, there were higher levels of CD11c-positive adipose tissue macrophages, as well as elevated levels of TNF-α and leptin compared to body mass index (BMI)-matched women without PCOS. Additionally, in visceral adipose tissue, catecholamines exhibited significantly increased lipolytic effects, leading to the release of fatty acids from visceral adipose tissue, which were closely linked to insulin resistance. On the other hand, some studies have indicated that there is no significant difference in the transport and mechanism of action of fatty acids and monoacylglycerol in PCOS patients, regardless of whether adipocytes are normal or abnormal in size (23, 24).

Presently, the majority of studies concentrate on investigating the mechanism of IR in obese patients with PCOS. However, the cause of IR in lean PCOS patients might be associated with the pro-inflammatory differentiation of macrophages triggered by abnormal fatty acid lipolysis in visceral fat. It is noteworthy that the locations of macrophage aggregation in the two categories of PCOS patients are not identical (25). In patients with polycystic ovary syndrome (PCOS), there are elevated levels of androgens and serum homocysteine. In mice with PCOS induced by dehydroepiandrosterone (DHEA), homocysteine leads to an imbalance of M1/M2-type macrophages in adipose tissue, causing a shift from M2-type to M1-type macrophages. This transition exacerbates DHEA-induced IR. Furthermore, the abnormally elevated insulin levels at this stage can stimulate androgen secretion from ovarian membranous cells, reduce insulin receptor autophosphorylation in ovarian granulosa cells, and further worsen the chronic inflammatory response and IR in the ovaries (26, 27).

T cells are a type of immune cell that matures in the thymus and plays a crucial role in specific immunity within an organism. They exhibit diverse subtypes and functions. CD4+ T cells contribute to maintaining the body’s immune response by recognizing major histocompatibility complex (MHC)-II-like molecules on the surface of antigen-presenting cells, subsequently activating and guiding other immune cells to the infection site. In the follicular fluid of patients with PCOS, the proportion of CD8+ T cells was notably lower compared to CD4+ T cells (28). CD4+ T cells play a more significant role in various clinical manifestations in PCOS patients, including IR. Based on their complex metabolic programming and cytokine production, CD4+ T cells can differentiate into distinct functional subpopulations. Among these, Th1 [producing interferon-gamma (IFN-γ)], Th17 (producing IL-17, IL-21, and IL-22), and Th2 (producing IL-4, IL-5, and IL-13) effector cells primarily rely on aerobic glycolysis for energy generation. In contrast, regulatory T cells (Treg) [producing IL-10, transforming growth factor β (TGF-β)] depend on oxidative phosphorylation driven by fatty acid oxidation (29–31).

In peripheral blood and adipose tissue of PCOS patients with metabolic abnormalities, T cells are the second largest immune cell population after macrophages, with significantly increased proportions of Th1, Th17, and CD8+ T cells and decreased proportions of Th2 and Treg cells (32–34). Metabolites of adipocytes can influence T-cell differentiation mediated by the T cell antigen receptor (TCR), and of the many soluble differentiation stimulators, fatty acids have been shown to be the strongest stimulators of Th1 differentiation (35, 36). T cells differentiated into pro-inflammatory Th1 cells produce IFN-γ, which induces macrophage differentiation towards the M1 phenotype. The aggregation of Th1 cells and infiltration of IFN-γ will recruit other immune cells, including macrophages, and increase the body’s inflammatory immune response (37). The above responses will be more pronounced in obese individuals. It was found that, in addition to its effects on immune cells, IFN-γ can directly induce IR in mature human adipocytes by taking charge of the activation of the janus kinase (JAK)/signal transducer and activator of transcription 1 (STAT1) pathway and inhibiting the expression of insulin signaling genes (GLUT4 and IRS1), lipid-forming genes (Perilipin, Lipoprotein Lipase, and Fatty Acid Synthesis Enzymes), and genes related to lipid storage (Recombinant Peroxisome Proliferator Activated Receptor(PPAG) and Lipocalin) (38). Insulin sensitivity was improved when the interferon gene was knocked out (39).

In the immunoregulation of T cells, the co-inhibitory receptors cytotoxic T-lymphocyte-associated protein 4(CTLA-4) and programmed cell death protein 1(PD-1) are co-expressed on effector T cells and participate in the dynamic balance of the immune response. The expression of PD-1 is maintained in T cells, which participates in the routine immune response of the body. In the acute phase, the expression of PD-1 decreases rapidly, and the body produces a rapid immune response. The proportion of T cells with elevated PD-1 expression in serum and follicular fluid was significantly higher in patients with PCOS than in normal patients, then metabolic disorders in patients with PCOS are associated with PD-1 expression (40). PD-1-overexpressing T cells are one of the subpopulations of the T-cell depletion phenotype (41, 42), and when in the presence of a PD-L1/PD-1 pathway blocking antibody, dendritic cells trigger the proliferation of syngeneic T cells and modulate T-cell subsets, which have elevated levels of IL-2 (43). This may suggest that in PCOS patients with abnormal T cells, PD-1 always plays a role in the relevant pathway and that high expression of PD-1 may inhibit T cell differentiation to Th2 type cells that secrete anti-inflammatory cytokines. In contrast, insulin has an anti-inflammatory effect on circulating immune cells, which may induce T cells to differentiate into an anti-inflammatory Th2 phenotype, and the dysregulation of the Th1/Th2 ratio and the PD-1-associated pathway may play a key role in resisting the effect of insulin on the immune response associated with T cells (44, 45).

CD4+ CD25+ Foxp3+ Treg cells were significantly lower in the peripheral blood of PCOS patients than in controls (34). FOXP3+ Treg cells can stimulate the production of IL-10 by macrophages through the secretion of IL-13, mediated by transforming growth factor-β. TGF-β and IL-10 act as the main anti-inflammatory cytokines in metabolism (46, 47). Smad4 and Recombinant Runt Related Transcription Factor 2 (RUNX2) genes are part of the TGF-β signaling pathway. The expression of these genes and related mRNAs increases during the development of hyperinsulinemia and IR in organisms (48). PCOS patients also differ from some obese patients in terms of hormone secretion levels. Estrogen helps limit inflammation, and in PCOS patients with hyperandrogenemia, the expression of the transcription factor B lymphocyte-induced maturation protein 1 (BLIMP1), which is androgen-dependent, is elevated. This leads to an immune response that brings about a new balance in the interaction between increased adipocyte inflammation and male-specific IL-33-producing stromal cells that actively recruit and locally expand Treg cell populations in a BLIMP1-dependent manner under the influence of sex hormones (49). In PCOS patients, there was a notable increase in Th17 cells, which predominantly secrete IL-17 to trigger neutrophilic inflammation, despite an overall decrease in Treg cells in the serum (34, 50). This indicates that the imbalance between Th17 and Treg cells in the context of abnormal insulin metabolism in PCOS patients is primarily due to the reduced levels of Treg cells.

Similar to T-cells, B-cells are classified into different subpopulations based on their distinct phenotypes, functions, and the primary cytokines they secrete. Presently, there are two primary categories: B-1 and B-2 cells. B-1 cells are predominantly found in organ cavities, mucosal tissues, and adipose tissues. They can be further categorized into CD5+B-1a and CD5-B-1b based on their CD5 expression (51, 52). B-1a cells, as innate B-like cells, are the primary producers of natural IgM antibodies in the body (53), and mainly function in the absence of antigens. In contrast, B-1b cells participate in T-cell-mediated immune responses to external antigens. B-2 cells are B cells traditionally derived from the bone marrow and migrate to lymphoid organs (54). They generate and concentrate specific antibodies, playing a crucial role in humoral immunity, and have the ability to differentiate into memory B cells and plasma cells. When B cells secrete anti-inflammatory factors like IL-10, and IL-35, they are collectively known as regulatory B cells (Bregs). These cells are not distinct subtypes from B-1 and B-2 cells but rather functional subtypes within these two classes of B cells.

In the context of IR driven by inflammatory adipose tissue, B-cells are the initial immune cells to accumulate, followed closely by T-cells, and eventually by macrophages (55). In individuals with PCOS experiencing low-grade chronic inflammation, the levels of anti-inflammatory IL-10 produced by Bregs were significantly lower compared to controls (56–58). Conversely, enhancing IL-10 expression or administering it for short durations in IR mice led to a notable increase in systemic insulin sensitivity. This effect was lost in the absence of IL-10 (59–61), indicating its role in inhibiting the differentiation of T cells into pro-inflammatory Th1 cells.

In patients with PCOS, the levels of IL-10 produced by B-1 cells decrease, leading to an increase in T-cell differentiation towards a pro-inflammatory phenotype, consequently promoting the development of insulin resistance originating from visceral adiposity (62–64). Additionally, B-2 cells have been demonstrated to have a pro-inflammatory function in the expansion of adipose tissue in rats consuming a high-fat diet (65). Moreover, in PCOS patients, this activation of B cells is closely linked to hyperandrogenism and the androgen receptor (66).

In line with mouse data, hyperandrogenemia also increases levels of IL-1β, IL-8, and IL-18 in PCOS patients (67). IL-8 functions as a pro-inflammatory factor that attracts neutrophils to adipose tissue, contributing to inflammation and IR (68). Furthermore, in obese PCOS individuals, there are changes in the levels of the pro-inflammatory cytokine leptin. Leptin triggers the phosphorylation of JAK2, STAT3, p38MAPK, and ERK1/2 in B-cells, decreases the expression of apoptotic factors, enhances B-cell survival, and promotes Th1 cell differentiation (69, 70). Limited research has focused on the pro-inflammatory mechanisms of B cells in insulin-resistant PCOS patients. Additionally, the association between B cells and macrophage polarization mechanisms in dysfunctional adipose tissue requires further exploration.

Dendritic cells (DCs) are antigen-presenting cells that play a role in both innate and acquired immunity. They are categorized based on morphology into conventional DCs (CDCs) and plasma cell-like DCs (PDCs) (71). DCs express surface markers like MHCII, CD11b, CD11c, and C-X3-C motif chemokine receptor 1, which are shared with macrophages. This similarity indicates that in dysfunctional adipose tissue, DCs function akin to macrophages (72, 73). Unlike many other immune cells that undergo substantial proliferation, DCs proliferate within tissues, albeit to a lesser extent (74, 75).

CDCs in adipose tissue can be subdivided into cDC1 (CD4- CD8α+ CD103+ CD205+ CD11b- CLEC9A+ XCR1+ CD24+ MHCII-) (76–78) and cDC2 (CD4+/- CD8α- CD205-CD11b+ CLEC4A4+ CD24+ MHCII+) (79, 80) based on descent. Both types of DCs increase in obese or inflamed tissues. cDC1 cells lacking MHCII are involved in cross-presentation to CD8+ T cells, leading to an expansion of CD8+ cells (81). The lack of MHCII expression in cDC1 and the decrease in the total number of CD11c+ cells enhance the body’s insulin sensitivity and lower the likelihood of IR. Nevertheless, this evidence alone does not imply a direct involvement of CD11c+ DCs in IR; CD11c+ macrophages also contribute to the reduction in inflammation levels (77).

Additionally, human monocyte-derived dendritic cells (moDCs) express CD14+ and are considered precursors of inflammatory dendritic cells (82). Conventional dendritic cell type 2 (cDC2) triggers CD4+ T-cell differentiation and exhibits a protective effect against adipose tissue inflammation. This effect induces IL-10 production through the activation of the Wnt/CD11b-catenin pathway in cDC2 cells [CD11c(hi) MHCII+ CD11b-], which typically express MHCII. Conversely, the previous pro-inflammatory function can be suppressed in dendritic cells [CD11c(hi) MHCII+ CD11b+] by activating the PPARγ pathway (83). Both mechanisms may contribute to IR in patients with PCOS. Moreover, the influence of serum estradiol levels on gonadotropins in PCOS patients impacts the maturation of CD11c+HLADR+ dendritic cells in human follicular fluid, consequently affecting IR levels (84). However, the precise molecular mechanism underlying this process remains unknown.

NK cells, which are innate lymphoid-like cells (ILCs) originating from the bone marrow, are widely distributed in various organ tissues and serve specific immune functions. Studies have shown that NK cells constitute 13% of immune cells in visceral fat and contribute to the inflammatory polarization of the immune response (85, 86). Inflammatory conditions predominantly regulate the local activity of NK cells through IL-12, IL-15, and IL-18 produced by dendritic cells and macrophages, with a particular focus on the extensively researched role of IL-15 (87–89).

IL-15 is highly expressed in the follicular fluid of PCOS patients and is positively correlated with serum testosterone levels. Treatment with IL-15 enhances the expression of Cytochrome P450 17A1 (CYP17A1) in granulosa cells, a key enzyme for androgen synthesis. Elevated levels of CYP17A1 lead to increased production of DHEA, a precursor for most androgens. Elevated androgen levels significantly contribute to inflammation and inflammation-induced IR (90).

In an inflammatory state, the expression of adipocyte NKp46 (NCR1) ligand is upregulated, and IL-15 binds to IL-15Rα on NK cell membranes, activating NK cells to produce IFN-γ. This promotes the polarization of CD4+ cells and macrophages towards an inflammatory state, contributing to the development of IR. Aggregated macrophages secrete large amounts of chemokines such as C-C motif chemokine ligand 3(CCL3), C-C motif chemokine ligand 4(CCL4), and chemokine CXC ligand 10 (CXCL10), which facilitate the recruitment of NK cells (91, 92).

Adipose tissue macrophage inflammation and IR can be improved by inhibiting NK cell function using neutralizing antibodies or through E4bp4 heterozygous knockout in mice (93). This is because inhibiting NK cell function leads to decreased expression of TNF-α and IL-1β, while increasing the expression of anti-inflammatory IL-10 and Arg1. Furthermore, leptin and lipocalin, linked to obesity, also regulate NK cell function. Short-term exposure to leptin enhances NK cell cytotoxicity and IFN-γ levels, whereas long-term exposure has the opposite effect. Conversely, lipocalin consistently suppresses NK cell function, irrespective of treatment duration (94, 95). This data indicates that in obese patients with PCOS, elevated leptin levels and/or decreased lipocalin levels activate NK cells, promoting the initiation and progression of inflammatory responses in adipose tissue, partially elucidating the development of IR in obese PCOS patients.

Due to significant phenotypic diversity and individual variances in PCOS patients, limited research has focused solely on NK cells, their cytokines, and their precise mechanisms.

Granulocytes are a type of leukocytes characterized by specific cytoplasmic granules. They are classified into major subgroups, including eosinophils, basophils, and neutrophils. In the complex inflammatory environment of PCOS patients, granulocytes are rapidly generated in the bloodstream, contributing to the immune response (96, 97). Specifically, the neutrophil-to-lymphocyte ratio (NLR) showed a positive correlation with HOMA-IR and serum insulin levels, irrespective of the obesity level in PCOS patients. This indicates a potential higher proliferation of granulocytes compared to lymphocytes in PCOS patients. Among the three types of granular leukocytes, neutrophil count and its proportion in leukocytes are commonly utilized as serum inflammation markers. Lourdes et al. observed that in PCOS patients with both hyperandrogenemia and hyperinsulinemia, the elevated white blood cell counts were primarily attributed to increased neutrophils. Moreover, glucose-lowering medications effectively suppressed the rise in neutrophils, thereby alleviating the inflammatory response in patients (98). Currently, it is believed that the mechanism of neutrophils affecting IR in PCOS is linked to myeloperoxidase (MPO). This belief stems from the higher MPO levels in leukocytes of PCOS patients compared to controls, with a more significant increase in the presence of IR (97, 99). However, due to the harsh culture conditions of granulocytes and the difficulty of detection, no authoritative study has yet revealed the specific mechanism of their role in IR in PCOS patients.

Overall, the mechanisms by which immune cells influence insulin resistance in PCOS patients are intricate and varied, necessitating further investigation into the functions of most immune cells ( Figure 3 ).

Tacrolimus (FK506) is an 822 kDa lipophilic macrolide antibiotic known for its potent immunosuppressive properties. It has been shown to inhibit the production of IFN-γ and IL-2 by activated T cells, as well as the aberrant expression of chemokine CXC ligand. This inhibition effectively suppresses the inflammatory response in target organs, such as the ovary and adipose tissue in murine models, thereby promoting ovarian ovulation and providing protection against IR (166–168).

Isotretinoin, a derivative of vitamin A, acts as an effective antiproliferative and antikeratinizing agent with certain immunosuppressive effects. A prospective clinical study conducted in Egypt investigated the impact of oral isotretinoin administration in patients with PCOS. The study reported a significant reduction in both free testosterone levels and acne scores following treatment. However, it also noted a considerable increase in serum triglyceride and cholesterol levels among the patients (169). Importantly, the study did not include measurements of insulin and glucose levels. These findings suggest that while isotretinoin may effectively modulate hormonal levels in PCOS patients, it does not appear to alleviate symptoms associated with metabolic disorders.

Mesenchymal stem cells (MSCs) significantly modulate the therapeutic immune response in a murine model of PCOS. Numerous studies have shown that MSC treatment markedly reduces serum expression of ovarian-specific genes and peripheral levels of the pro-inflammatory cytokine TNF-α in mice with PCOS. This effect may be mediated by the anti-inflammatory cytokine IL-10 (170–172). Peripheral blood flow cytometry analysis by Lamei Cheng et al. revealed that MSC treatment restored the proportions of macrophages and neutrophils in both peripheral blood and spleen to normal levels in the PCOS mouse model. Additionally, the ratio of pro-inflammatory M1 to anti-inflammatory M2 was appropriately adjusted. Variations in donor origin significantly affect the immunomodulatory effects of MSCs. Specifically, adipose-derived MSCs from obese or IR individuals show a markedly reduced ability to downregulate inflammatory factor expression and suppress CD4+ T cell activity. This reduced efficacy may result from intrinsic abnormalities in MSC function related to insulin resistance (173, 174). Currently, stem cell therapy targeting PCOS patients with IR, along with the underlying mechanisms affecting the immune system and stem cell abnormalities, has not progressed to the clinical research phase.

The clinical treatment strategy for IR induced by PCOS is comparable to that for type 2 diabetes. Lifestyle management is the preferred approach. Engaging in appropriate exercise and following a low glycemic index (LGI) diet can reduce waist circumference, total testosterone, low-density lipoprotein (LDL), fasting insulin, LDL cholesterol, triglycerides, and total cholesterol (175). The LGI diet decreases levels of growth hormone-releasing peptide while increasing glucagon levels. Physical activity and vigorous aerobic exercise enhance insulin sensitivity and androgen levels in patients with PCOS. Excessive fatty acid consumption and stabilization of hormone levels can effectively alleviate immune dysregulation in patients with PCOS. Lifestyle management is now widely promoted globally. Medications used as adjunctive treatments can effectively manage long-term complications.

Metformin (Met) is currently the primary therapeutic agent used for insulin sensitization in individuals with PCOS, while glucagon-like peptide-1 receptor agonists (GLP-1 RAs) are commonly employed in the treatment of T2DM. A study by Liao et al. investigated the efficacy of combining GLP-1 RAs and Met in reducing IL-6 and other molecules related to the inflammatory immune response, as evaluated through plasma proteomics (176). Additionally, Met has been shown to affect the levels of 11 cytokines in patients with PCOS, and its therapeutic efficacy is closely linked to androgen levels in these individuals (177).

The selection of pharmacological agents is crucial for improving insulin sensitivity and managing other metabolic disorders in patients with PCOS. As shown in the previous section, a significant association exists between the immune system and IR in patients with PCOS. Therefore, selecting medications that modulate inflammatory cytokine activity or inhibit the recruitment and proliferation of pro-inflammatory immune cells may offer a promising therapeutic strategy for future research.

In addition to medications, the incorporation of supplements may theoretically enhance the hypoglycemic effects of Met (e.g., sage, curcumin, quercetin, inositol, and various herbs) (178). However, aside from potential synergistic or additive effects with Met, no other known herb-drug interactions have been documented (179). Future therapeutic studies should investigate potential interactions between herbs or nutrients and drugs, as well as any immediate or long-term risks associated with these interactions.