HIV-1 infection results in systemic immune activation, which depends on virus replication and/or microbial translocation, and is resolved to an extent by antiretroviral therapy[5
]. Systemic immune activation drives viral replication creating a cycle of T cell attrition. When combined with viral effects on thymic T cell output, dissecting the contribution of each disease variable to immune activation poses significant challenges. Our studies show that during the natural history of pediatric HIV-1 infection, T-cell activation and CD4 T-cell attrition are related to levels of viral replication depending on age/length of infection. In contrast, macrophage activation is related to both microbial translocation and effects of HIV-1 infection that persist even when viremia is suppressed below limits of detection. Sustained immune reconstitution by ART, independent of viral outcomes, induced concordant attenuation of CD4 and CD8 T cell activation[29
], even though microbial translocation or macrophage activation persisted indicating that systemic immune activation in T cell versus monocyte/macrophage compartments can be dissociated based on distinct mechanisms.
While viral replication is the principle factor driving lymphocyte activation, microbial translocation is also implicated in systemic T cell activation in some, but not all, natural history studies of HIV-1 infection[5
]. In contrast our study of perinatally-infected children found no association between microbial translocation and T cell activation or immune deficiency pre-ART, highlighting the complex relationships between different causes and consequences of immune activation. Following ART, reconstitution of CD4 T cells and increased TREC occurred in our cohort despite persistent microbial translocation indicating that thymic output, a major contributor to post-ART lymphocyte reconstitution[29
], is independent of LPS-driven inflammation. Failure of ART to attenuate microbial translocation and LPS levels in children, unlike ART treatment of HIV-1 infected adults[5
], may reflect irreversible intestinal fibrosis induced by early viremia during infancy[45
]. HIV-1-infection during infancy, compared to infection during adolescence or adulthood may have different effects on MALT due to normal processes of gut remodeling that occur during growth[46
] when intestinal permeability is increased and adaptive immunity is still developing. Indeed, transient microbial translocation found in our study of healthy and HIV-1-infected infants resolved after 2 years of age only in healthy children, indicating that HIV-1 disrupted normal gut development. It would be interesting to further investigate the nature of microbial translocation in early infancy versus older HIV-1-infected children to determine if disruption of gut epithelial and endothelia tissues differs by age and illness. Microbial translocation, as a consequence of HIV-1 pathogenesis in adults, is inversely correlated with EndoCAb levels[5
]. In our study, EndoCab levels were reduced among HIV-1 infected children, but no relationship between EndoCab and LPS levels was apparent. Developmental regulation of natural antibody production could impair the ability of perinatally-infected children to neutralize and clear circulating LPS during early infection.
The LPS detected in plasma of HIV-1 infected subjects is biologically active and inflammatory in vivo
, reflected by the link with monocyte/macrophage activation, although healthy infants with similarly elevated LPS levels displayed no concomitant increase in monocyte/macrophage activation. HIV-1 might function as a co-factor for LPS-induced inflammation in infants. The overall extent of monocyte/macrophage activation found with HIV-1 infection could reflect direct activating effects of HIV-1 on monocytes[6
] or priming by HIV-1, which sensitizes macrophages to subsequent activation with toll-like receptor [TLR] ligands including LPS[50
]. Whether sCD14 detected in plasma is derived from monocytes or differentiated macrophages is unclear, but both scenarios are plausible. In vivo
, macrophages are efficient targets for productive HIV-1 infection whereas monocytes are rarely infected[51
], although binding of HIV-1 envelope proteins to CCR5 on monocytes induces cell signaling that enhances monocyte survival[53
] and may induce activation and shedding of sCD14.
Lack of monocyte/macrophage responsiveness to LPS in healthy infants is characteristic of normal immune development[26
]. For example, in atopic allergy, an immune hypersensitivity disorder, sCD14 levels are lower in neonates versus older children, even though neonates have increased frequency of CD14+
]. Breast-feeding is another variable that regulates inflammatory versus tolerogenic responses to microbial translocation. Secretory IgA in breast milk can have significant effects on gut microbial flora through alterations in colonization and prevention of trans-mucosal sepsis, and other soluble milk components can mediate anti-inflammatory effects[28
]. None of the neonates in our study was breast fed, which may itself contribute to immune activation. Clearly, systemic monocyte/macrophage activation is less pronounced during childhood and HIV-1 infection significantly disrupts normal immune homeostasis. Although ART for HIV-1-infected subjects in this study improved health with significant increases in height and weight and decreased prevalence of AIDS-defining illnesses[24
], inability of therapy to resolve either microbial translocation or monocyte/macrophage activation may contribute to accelerated immune senescence with increased long-term morbidity and mortality from non-AIDS related inflammatory conditions[55
CD27, a member of the tumor necrosis factor receptor family and its ligand, CD70, are predominantly expressed by lymphocytes[56
]. Lymphocyte activation in both the T cell and B cell compartments results in high levels of the soluble protein in the plasma and can be used to measure systemic immune activation in HIV-1 infection[56
]. The divergence in sCD27 levels in post-ART viral success versus viral failure subjects may be linked to qualitative differences in HIV-1 phenotype and co-receptor utilization. HIV-1 isolated from subjects who suppress viremia primarily use only CCR5 co-receptor for target cell entry, while HIV-1 from viral failure subjects use both CCR5 and CXCR4 co-receptors before and after ART[33
]. Persistently elevated sCD27 demonstrates that replication of drug-resistant HIV-1 induces some component of lymphocyte activation in spite of improved CD4 T cells counts and normal immune function[24
], indicating that systemic immune activation and its consequences are complex and multifactorial.
Discordant outcomes following ART represent an important paradigm in HIV-1 pathogenesis, which may be modeled by high levels of SIV replication in its natural host[18
]. However, our study demonstrates fundamental differences between discordant therapy outcome in HIV-1 infection and natural SIV infection of sooty mangabeys in that CD4 T cell pathogenesis is uncoupled from microbial translocation in HIV-1 infection. In sooty mangabeys, viremia fails to induce intestinal fibrosis or microbial translocation which is related to the overall lack of CD4 T cell pathogenesis or immune deficiency in this model[5
]. In contrast, HIV-1-infected children experience increased naïve CD4 T cell counts and TREC levels after ART regardless of viral outcome, suggesting that persistently elevated LPS two years into therapy does not adversely impact thymic output[29
]. Post therapy viruses do not induce the same degree of T cell activation as wild type virus and are associated with higher TREC levels. In the discordant response group, post therapy drug resistant viruses are less fit for replication in the thymus resulting in a high proportion of naïve T cells in the peripheral blood[59
]. Thus, improved CD4 T cell frequency in post-ART discordant response subjects is more likely a consequence of attenuated viral pathogenesis than changes in gut pathogenesis or microbial translocation. Similar to adults who continue failing ART regimens[22
], discordant response in children and adolescents was associated with evidence of CD4 T cell declines by 96 weeks, although the proportion of CD45RO CD4 T cells failed to increase, an indication that robust thymic output persisted.
Our finding that microbial translocation is not a major factor in HIV-1 immunodeficiency in infants, children and adolescents is similar to a study of HIV-1 subtype A and D infections in African adults, who progressed to AIDS in the absence of HIV-1-specific microbial translocation and chronic HIV-1-associated inflammation[7
]. Nonetheless microbial translocation is clearly an important component of innate immune inflammation in some HIV-1-infected subjects. In our study, both outcome groups developed post-ART declines in sCD14 levels, but monocyte/macrophage activation failed to normalize in either group, even following two years of ART, indicating that microbial translocation contributes to sustained macrophage activation that can be independent of high level viral replication. Prolonged systemic monocyte/macrophage activation may contribute to HIV-1-associated inflammatory conditions including dementia, increased risk for cardiovascular events and thrombophilia or enhanced angiogenesis in malignancy[13
]. The extent of monocyte/macrophage activation and resulting inflammatory conditions may differ in perinatally-infected children compared to infected adults due to greater infectivity and HIV-1 replication in neonatal or cord blood-derived monocyte/macrophages[61
]. Considering that perinatally-infected neonates will likely survive several decades with ART, the implications of persistent microbial translocation and monocyte/macrophage activation should be a major focus of investigation in coming years.