We have described alterations in the homing of CD4+ T cells to the gut that could contribute to the lack of mucosal immune reconstitution in HIV-infected individuals despite cART. CD4+ T cells in the small intestine mucosa of the subjects studied remained depleted, despite the subjects being on effective cART for more than 5 years. This is in marked contrast to a substantial restoration of their blood CD4+ T cell count. The persistence of microbial translocation and systemic inflammation in these subjects underlines the importance of restoring an efficient mucosal immune barrier.
The immune inductive sites of the GALT are better reconstituted than effector sites in response to cART (24
). But unlike the ileum and colon, the mucosa of the upper small intestine is devoid of lymphoid follicles. We have focused on CCR9+
T cells trafficking to the jejunum mucosa. This allows us to assess the immune reconstitution of the effector sites of the gut during cART, without bias due to the presence of immune inductive sites, as is the case for the ileum or colon mucosa.
We found that a significant proportion of CCR9+
T cells are in the circulation of HIV-infected individuals rather than localizing in the gut, as they are in uninfected controls. Similarly, the distribution of CCR9+
T cells between the blood and small intestine compartments appears to be the inverse in inflammatory bowel diseases, as in celiac and Crohn’s diseases, with the frequency of CCR9+
T cells in the peripheral blood being markedly increased and that in the small intestine mucosa being reduced (31
). It has been suggested that the preferential apoptosis of CCR9+
T cells or the downregulation of CCR9 expression after T cell activation in the small intestine mucosa may explain the reduced frequency of CCR9+
T cells in the gut mucosa during celiac or Crohn’s diseases. The ex vivo activation of T cells downregulates CCR9 expression (31
). However, we found no expansion of the CCR9–
T cell subset accompanying the reduction of the CCR9+
T cell subset in the gut of HIV-infected individuals (data not shown). Some downregulation of CCR9 on T cells could have occurred during tissue processing, but it cannot account for the difference we found between HIV-infected individuals and healthy individuals. The preferential activation-induced death of CCR9+
T cells could contribute to their depletion in the gut mucosa. But our data better support a mechanism involving a lack of recruitment of CCR9+
T cells to the gut, as the amount of CCL25 in the small intestine mucosa of HIV-infected individuals was much lower than that in uninfected controls. Moreover, the amount of CCL25 appears to be positively correlated with the frequency of CCR9+
T cells in the gut mucosa but inversely correlated with their frequency in the peripheral blood. The expression of CCL25 is altered in Crohn’s disease, with patchy increases in areas of lymphocyte infiltration but reduced amounts in inflamed and ulcerative areas of the small intestine mucosa (31
). By contrast, we found a diffuse reduction in CCL25 in the jejunum mucosa of HIV-infected individuals. We detected CCL25 expression by immunohistochemistry in both the crypt and villous epithelial cells. Some published studies found more CCL25 in crypt epithelial cells (4
), while others found more CCL25 in villous cells (33
). Ericsson et al. isolated epithelial cells from crypt and villous regions by laser capture microscopy and found high amounts of CCL25
mRNA in both crypt and villous enterocytes (34
). A reduction in the amount of CCL25
mRNA has also been observed in macaques lymph nodes during SIV infection (35
). Epithelial cells are the main source of CCL25 in the small intestine mucosa. The reduced expression of CCL25 could thus be due to persistent enterocyte damage in HIV-infected individuals despite cART. This, in turn, could impair the CCL25-mediated recruitment of CCR9+
T cells to the gut, setting up a vicious circle that prevents efficient mucosal reconstitution.
Gene expression profiles of gut mucosal tissue have revealed increased activity of the genes involved in inflammation and apoptosis in individuals with poor CD4+
T cell restoration in the gut, while the genes involved in mucosal repair and regeneration are more active in those having an efficient immune restoration (36
). Residual HIV-1 replication in the gut despite cART could result in the intestine mucosa being persistently inflamed. Some virus proteins might have bystander effects, notably gp120 signaling through GPR15/Bob on the enterocytes could lead to their apoptosis (37
). Intraepithelial T cells bearing the Fas ligand could also induce Fas-mediated apoptosis of the enterocytes. The resulting damage to enterocytes could be responsible for the reduced expression of CCL25 in the small intestine mucosa and the associated defective homing of CD4+
T cells to the gut in HIV-infected individuals.
Other mechanisms are probably involved in this incomplete mucosal immune reconstitution, notably, in the colon, in which the CCL25-CCR9 axis does not play a major role. The chemokines CCL28, CCL20, and CXCL12 and the ligands of CXCR3 could play a role in the homing of T cells to the gut, but little is known regarding a potential dysfunction of this chemokine network in the setting of HIV infection (39
). The depletion of CD4+
T cells in treated HIV-infected individuals appears much more pronounced in the small intestine than in the colon (41
). Thus, while the defect in the CCL25-CCR9 axis is probably not the sole mechanism involved — as it cannot explain the lack of immune reconstitution of the colonic mucosa — its impairment could contribute significantly to the particular depletion of CD4+
T cells observed in the small intestine mucosa.
The disrupted integrity of the gut mucosal barrier allows translocation of microbial products from the gut lumen into the bloodstream during HIV-1 infection (20
), as in inflammatory bowel diseases (44
). HIV-infected individuals have been reported to have increased plasma levels of I-FABP and LPS, reflecting ongoing enterocyte death and bacterial translocation. Further, high levels of sCD14, a marker of monocyte activation after stimulation by LPS, have been associated with an increased risk of mortality in HIV-1 infection (18
). Th17 cells play a critical role in the immune defenses of the gut mucosa. We found that most gut-homing Th17 cells are within the CCR9+
T cell subset and that the defective homing of this subset to the gut is associated with microbial translocation. The lack of recruitment of Th17 cells to the gut could thus be involved in the persistence of a “leaky” intestinal mucosal barrier.
In conclusion, we find that the defective homing of CCR9+β7hi CD4+ T cells to the gut could impair mucosal immune reconstitution in treated HIV-infected individuals. A vicious circle could arise among mucosal inflammation, enterocyte damage, reduced CCL25 expression, and defective homing of CCR9+β7hi CD4+ T cells to the gut. The lack of recruitment of CCR9+β7hi CD4+ T cells to the gut was associated with persistent microbial translocation and systemic T cell activation despite cART. Monitoring the frequency of circulating CCR9+β7hi CD4+ T cells can provide a surrogate marker of poor immune reconstitution of the gut mucosa in treated HIV-infected individuals.