Fetal airways are essentially devoid of lymphocytes, they are seeded from birth, and lymphocytes increase as a proportion of airway cells over the first few years of life (48, 54). There is a relative paucity in CD4+ cells (46, 50), and memory T cells are less abundant in infant lungs than in adults, though they are more abundant in the lungs than many other tissues (51). Tregs are relatively abundant in pediatric tissues and may have a higher suppressive capacity than those from adults (28, 51) and a transient increase in regulatory T cells, associated with microbial colonization, protects from hyperresponsiveness to allergen (35). A failure of regulation may underlie excessive inflammation in infection, as in RSV bronchiolitis (43), and RSV infection in early life can increase susceptibility to allergic inflammation in the mouse model through an impairment of regulatory T cells (4, 55). CD8+ T cells in the lung correlate with disease severity in infants with respiratory failure due to respiratory viral infection (52) and in neonatal mice infected with RSV, a CD8+ T cell epitope hierarchy emerges, which is distinct to that of adults (56). Distinct phenotypes of adaptive lymphocytes are found in early life. A subset of Th cells in human cord blood produce the neutrophil chemoattractant interleukin-8 upon activation (57) and, during RSV infection, a regulatory phenotype in the neonatal B cell compartment may dampen protective immunity (58).

There is some evidence that neonatal T cells have the capacity to mount adult-like protective responses to lung infection. Adoptive transfer of neonatal CD4+ T cells into Pneumocystis carinii-infected adult SCID mice allowed for adult-level pathogen clearance and cytokine production (59, 60), suggesting that the neonatal environment in the lung influences T cell responses. This may be due in part to the function of neonatal antigen-presenting cells. Neonatal mouse lungs contain relatively fewer conventional DCs (cDCs), which are immature and poorly functional (56, 61, 62), although mature functions ex vivo have been reported (63). During neonatal RSV infection, migratory cDCs are dominated by CD103+ DCs, while the CD11b+ contribution increases with age (64). These CD103+ DCs are phenotypically immature and poorly functional (65), and this may influence the magnitude and epitope hierarchy of the CD8+ T cell response (64–66), although these are also influenced by T cell intrinsic differences and regulatory T cells (56, 67). As well as stimulating protective responses, lung DCs in neonates must promote tolerance to harmless environmental antigens. CD11b+ cDCs in the lung induce Th2 responses to allergens, but transiently express high levels of PD-L1, which promotes tolerance, following acquisition of the microbiota (35, 68). In contrast to murine studies, the relative frequency of different DC subsets in the human lung appears to be relatively stable over the life course (53).

In the murine neonatal lung, potent IFN-α-producing pDC cells are scarce (61), and there is limited recruitment of pDCs and IFN-α production following RSV infection (69).

Lung resident macrophages, which include AM and the less well-characterized interstitial macrophages (70–72), are an important component of the first line of defense in the lung. In the steady state, AMs remove debris and maintain a tolerogenic environment; during infection, they secrete proinflammatory cytokines and contribute to pathogen clearance; and after infection, they aid resolution of inflammation (45). AMs are the predominant cell type in the neonatal airway, they appear in the alveolar compartment from just before birth and throughout the first week of life, and are relatively abundant and self-renewing, persisting for at least 11 weeks in mice (47–50, 73, 74).

Stimulation of cultured cells has been used to interrogate the relative antimicrobial functions of neonatal and adult AMs. LPS stimulation of rodent or ovine AMs results in similar or even enhanced upregulation of TNF-α and CXC-chemokines in neonatal compared to adult cells (75–77), though others demonstrated a reduced translocation of NF-κB to the nucleus of AM from neonatal mice (78). Enhanced phagocytosis by neonatal compared to adult rat AM has been observed (75), but others have reported impaired phagocytosis and subsequent killing of yeast particles in neonatal rhesus monkey AMs; and impaired phagocytosis of opsonized red blood cells in neonatal rat AMs in comparison to adults (79, 80). In a murine model of Pneumocystis infection, neonatal AMs were delayed in their expression of activation markers in vivo in comparison to adults (81). Similarly, during murine neonatal RSV infection, there was reduced and delayed AM activation compared to adult infection (82), but intranasal IFN-γ was able to promote AM maturation (82). Little is known about responses in human infant AMs. Cultured cells obtained by bronchoalveolar lavage from infants <2 years of age produce lower IL-1 and TNF-α following LPS stimulation compared with cells from children aged 2–17 (54). The apparent contradictions in the data on AM function in early life may reflect differences in the species, age, experimental conditions, and assays used. Various macrophage functions are likely to mature at different rates. Neonatal and adult AMs are likely to behave differently in their respective lung environments, which is a limitation of these in vitro studies.

The respiratory epithelium is the principal site of replication of respiratory viruses. It is in close communication with AM and acts an immune sentinel producing inflammatory mediators, such as type I and III interferons, mucus, and antimicrobial proteins (45, 83). Relatively little is known about the immunological functions of the airway epithelium in early life. In cultured tracheobronchial epithelial cells from Rhesus macaques of different ages (infant, juvenile, and adult), IL-8 production on exposure to LPS positively correlated with age (84). Furthermore, epithelial cells from juveniles housed in filtered air produced higher cytokine responses than those in conventional housing suggesting the microbial richness of the environment may influence epithelial responsiveness. The same group demonstrated that infant Rhesus monkey primary epithelial cell cultures are more permissive for the H1N1 influenza virus than those from adult airways, while producing less IL-1α (85).

In humans, type I IFNs are detected at only low levels in the airways of RSV-bronchiolitic infants. This may be due to inhibition of the host anti-viral response by the viral non-structural proteins but alternatively may reflect the timing of sampling, and an IFN-induced gene signature is detectable in blood (86–88). Pediatric nasal and airway epithelial cells cultured from bronchial brushings are readily infected with RSV (89–91) and poor induction of type I IFNs by RSV is reflected in these cultures (92, 93). Instead, the type III interferon IL-29 (IFN-λ) is detected both in the airways of bronchiolitic infants and in cultures of RSV infected airway epithelial cells, and IL-29 pretreatment of cultured epithelial cells attenuates RSV growth (92, 93). Epithelial cells are probably a key source of inflammatory cytokines in respiratory tract secretions of infants with acute RSV (92, 94, 95), including the type-2 immunity promoting cytokine IL-33 (96). The cells used in many in vitro experiments on pediatric respiratory epithelial cells were originally taken from the conducting airway and data surrounding lower airway and ATII cells in early life is even sparser.

Antimicrobial proteins are a first line of defense at barrier sites and are produced primarily by epithelial cells and innate leukocytes, particularly neutrophils (97, 98). In the lung, they include surfactants as well as S100s, β-defensins, and cathelicidin and they may provide protection against important infant respiratory infections, including RSV (99–102). Cathelicidin has direct antiviral activity against RSV, can prevent infection in vitro and in vivo and in children hospitalized with bronchiolitis, those with low serum cathelicidin were significantly more likely to have RSV infection and a longer hospital stay (97, 103–107).

Neonatal murine lungs show no quantitative deficiency in γδ T cells as a proportion of CD3+ T cells (61, 108). Exposure to allergen in neonatal mice can stimulate innate ILC2 lymphocytes, a major source of type 2 cytokines (109). Colonization by the microbiota in neonates protects against the accumulation of potentially pro-inflammatory mucosal iNKT cells in the lung and gut (110). Colonization of the gut of neonatal mice can also lead to intestinal DC mediated upregulation of CCR4 on IL-22 producing ILC3, which allows their migration into the lungs of neonatal mice, and promotes protection against bacterial pneumonia (111).

Recruitment of innate leukocytes and, in particular, neutrophils, is likely to play an important role in the innate response to infection in the neonatal lung following microbial recognition. Both TLR4 gene and protein expression are present in the murine lung in the fetus and increase with age through to adulthood (112, 113). TLR2 expression is also present in the human fetal lung and increases with gestational age (114). It appears that there is an immaturity of chemokine production at baseline in the respiratory mucosa. Expression of CXCL2 is low in neonatal mice compared with adults (115) and in uninfected infants (newborn to 18 months), the concentration of IL-8 in nasal washes positively correlates with age (116). There is a dramatically reduced and delayed neutrophil influx in neonatal lung in response to administration of LPS or bacteria in comparison to adult animals (75, 117–119). In the neonatal murine lung, infection with the paramyxovirus Sendai virus results in a minimal early influx of neutrophils and low production of pro-inflammatory cytokines compared with the adult lung; similarly in murine RSV infection, early pro-inflammatory cytokine production is impaired (108, 115). Diminished recruitment of neutrophils may also be due to an impaired chemotaxic ability of infant neutrophils (25, 120, 121).

In severe RSV bronchiolitis in infants, neutrophils can account for the majority of cells recovered from the airways, associated with increased neutrophil elastase (122–125) and IL-8 (94, 126), although others have reported a lower inflammatory cytokine response in infants with severe vs mild RSV bronchiolitis (127). There is a considerable influx of neutrophils into S. pneumoniae-infected lungs of neonatal and adult mice, with the neonatal influx even occurring at a lower bacterial dose (128). It is unclear under what circumstances the neonatal lung will produce an equivalent or exacerbated inflammatory response compared to that of adults, whether this simply requires a high level of stimulation or whether additional factors are involved.