Effect of α4β7 blockade on intestinal lymphocyte subsets and lymphoid tissue development

Integrins are dimeric cell surface molecules composed of α and β subunits that promote cell–cell interactions and play important roles in the immune system. Within the integrin family, α4β7 displays relative specificity for the mucosal immunity and has multiple functions including lymphocyte homing, intestinal lymphoid tissue formation, and promoting inflammatory responses.1–6 Blockade of α4β7 is an appealing therapy for intestinal inflammatory diseases, as the effects of blocking this pathway will largely be limited to mucosal sites. While diverse functions for α4β7 in intestinal immunity are well described, the essential nature of α4β7 in these roles is less clear, and blockade of these essential functions could result in untoward effects of this therapy including inhibiting the homing of nonpathogenic lymphocytes and the formation of homeostatic lymphoid tissues. Mucosal vascular addressin cell adhesion molecule (MAdCAM-1) is the binding partner for α4β7,1 and is expressed on high endothelial venules (HEVs) and postcapillary vessels in the intestine. In contrast, α4β7 is expressed by diverse populations of hematopoietic cells including lymphocytes, natural killer cells, monocytes, and lymphoid tissue inducer (LTi) cells.5–9 A role for α4β7/ MadCAM-1 interactions in lymphocyte trafficking to the intestine is widely accepted; however, the lymphocyte subsets, cellular compartments, and conditions in which α4β7 plays an essential role, and therefore would be affected by α4β7 blockade, are less clear. Early studies using adoptive transfer of labeled lymphocytes identified a role for α4β7 in lymphocyte homing to Peyer’s patches and mucosa in the uninflamed intestine.2 Later studies of using genetically modified mice confirmed that in the absence of β7 in the uninflamed intestine, Peyer’s patches had diminished cellularity and mucosal lymphocyte populations were decreased.10 However, these studies were performed prior to the appreciation of the diversity of lymphocyte subsets, and consequently information regarding the essential nature of α4β7 for homing of specific lymphocyte populations in the uninflamed intestine is largely lacking. Moreover, blockade of the α4β7/MadCAM-1 pathway has had variable effectiveness in animal models of intestinal inflammation,3,11–14 which could be explained by differences in the specificity of the blockade strategy, or the homing mechanisms used by lymphocyte subpopulations playing a pathogenic or protective role in each model. In addition to the above effects, α4β7 blockade may affect the development of intestinal lymphoid tissues during inflammation. α4β7/MadCAM-1 interactions play a critical role in the development of isolated lymphoid follicles (ILFs),6 or intestinal lymphoid aggregates, in the uninflamed adult intestine. During pathogenic inflammatory diseases, including inflammatory bowel disease (IBD), increased numbers of lymphoid aggregates are also seen,15–18 and the above observations suggest that their development may also be dependent on α4β7/MadCAM-1 interactions. During physiologic inflammation, ILFs mediate homeostatic functions in intestinal immunity; conversely, the function of lymphoid aggregates during chronic intestinal inflammation is less clear and may be pathogenic, as these lymphoid aggregates have been proposed to be the sites containing the earliest manifestations of IBD.19,20 Thus, α4β7 blockade could affect lymphoid aggregate development during uncontrolled intestinal inflammation, resulting in either detrimental or beneficial effects. In order to better understand the processes lost in the absence of α4β7 function we investigated the effect of α4β7 blockade and β7 deficiency on lymphoid tissue development and lymphocyte subpopulations in the uninflamed and inflamed intestine. In contrast to current perceptions, we observed that T-lymphocyte populations in the diffuse lamina propria and lymphoid aggregates were unaffected in the absence of α4β7 function in normal mice and in the acute injury dextran sodium sulfate (DSS) colitis model. Concordant with prior observations we observed that lamina propria B-lymphocyte populations and the development of B-lymphocyte containing lymphoid aggregates were both affected by loss of α4β7 function in the normal intestine and in the acute injury DSS colitis model. Furthermore, we observed that Foxp3+ T-lymphocytes, like B-lymphocytes, were dependent on α4β7 function to localize to lymphoid aggregates. In contrast to the above observations, T-lymphocyte trafficking in T-lymphocyte-mediated colitis model was dependent on α4β7 function. Anti-α4β7 blockade did not alter intestinal T-lymphocyte populations or ameliorate acute injury DSS-induced colitis, but did improve chronic T-lymphocyte-mediated colitis as evidenced by decreased T-lymphocyte infiltration into the intestine and decreased production of inflammatory cyto-kines. These observations demonstrate differential dependence on α4β7 by intestinal lymphocyte subpopulations and suggest that α4β7 blockade may be a more targeted therapy than previously appreciated by selectively blocking T-lymphocyte trafficking to the intestine during chronic T-lymphocyte-mediated inflammation.