We used two mouse models of bacterial-induced colitis to study the effects of blocking IL-7Rα on IBD: a T- and B-cell-sufficient strain (Mdr1a
−/−) and a T- and B-cell-deficient strain (Rag2
−/−). These models were used to circumvent the confounding variable of IL-7-dependent expansion of T cells as occurs in colitis models that rely on adoptive transfer of T cells into a lymphopenic environment. Our results show that IL-7Rα blockade can ameliorate bacterial-induced colitis, and that this effect involves not only T cells but also innate immune cells such as macrophages, DC, and NK cells. Successful treatment of colitis with an anti-IL-7Rα antibody was associated with decreases in T-cell and non-T-cell populations, as well as a reduction of inflammatory cytokines and chemokines. Our results confirm that IL-7Rα+ T-cell-mediated activities are likely key players in IBD. However, our results differ from some reports in that we demonstrate that IL-7R-mediated activity on non-T and non-B cells also contributes to experimental colitis.
In TCRα−/− mice and T-cell-transfer-induced colitis, high expression of IL-7Rα on T cells is associated with colitis [9, 10, 15], and IL-7-producing bone marrow cells may harbor colitogenic memory CD4+ T cells . The use of a toxin-conjugated anti-IL-7Rα antibody (A7R34) by Yamazaki and colleagues suggests that selective elimination of IL-7Rαhigh CD4+ LPL is required to treat colitis . In our studies with Mdr1a
−/− mice, we noted that all T-cell subsets in draining MLN were decreased with anti-IL-7Rα M595 treatment, indicating that decreasing total T cells (which likely included CD4+IL-7Rαhigh cells) was in large part responsible for the lack of colitis seen in anti-IL-7Rα M595 antibody-treated mice. We did not detect increased colonic expression of IL-7Rα mRNA in Hb-infected Mdr1a
−/− mice. The apparent discrepancy between our results of reducing total T cells vs. that of other’s work specifically eliminating IL-7Rαhigh CD4+ T cells may be due to the different model systems. In our models, Hb is driving chronic inflammation by both adaptive and innate cells, whereas the adoptive transfer models are skewed towards promoting IL-7Rαhigh T-cell expression and expansion. For example, in the 500 μg M595-treated Hb-infected Mdr1a
−/− mice, we saw a greater decrease in total CD4+ vs. CD8+ T cells, as well as in naïve vs. activated/memory CD4+ and CD8+ T cells. Naïve T cells express high levels of IL-7Rα, whereas activated T cells down regulate IL-7Rα . Our FACS analysis did not distinguish between activated effector cells and memory cells, but this total population was less affected by IL-7Rα blockade than naïve T cells, perhaps due to lower expression of IL-7Rα. Reducing the pool of naïve T-cells may help control colitis by removing cells that would become activated or develop into memory cells following exposure to Hb. Regardless of whether anti-IL-7Rα efficacy is due to specifically decreasing IL-7Rαhigh or IL-7Rα+ T cells in general, the combined effect of reducing both naïve and activated/memory T cells was associated with reduced colitis in Mdr1a
In Hb-infected Mdr1a
−/− mice, the lower dose of anti-IL-7Rα M595 (50 μg) was as effective in preventing colitis as the higher dose (500 μg), despite the lower dose having less of an effect on reducing T-cell numbers (in MLN). In fact, the lower dose preserved ratios of CD4+ to CD8+ and naïve to memory CD4+ cells in MLN similar to control levels. In the colon however, the reduction in CD4 mRNA expression was the same with both doses of M595. The lower dose of M595 was also effective in reducing inflammatory mediators as measured in colon and serum, indicating that a relatively low dose of M595 can also inhibit the function of T cells (and likely non-T cells). The mechanism by which M595 antibody decreases cellularity is likely due more to population decay by inhibiting survival signals [18, 19] rather than direct depletion of target cells. However, antibody-dependent cell-mediated cytotoxicity (ADCC) measurements of M595 showed it to be weakly lytic (Amgen Inc.; data not shown). It would be interesting to perform these colitis studies with a re-engineered anti-IL-7Rα antibody that does not have the confounding variable of ADCC. Further dose–response studies would be needed, but these results suggest there may be a therapeutic window for anti-IL-7Rα in inflammatory disease settings whereby T-cell function would be controlled without significant lymphopenia.
Given the significant alterations of chemokines and cytokines associated with innate immune cells in the serum and colon in the Mdr1a
−/− model, we explored anti-IL-7Rα treatment in a Rag2
−/− model of colitis. Amelioration of disease with anti-IL-7Rα treatment in the Rag2
−/− colitis model suggested that IL-7Rα expressing non-T cells, including macrophages, DCs, and NK cells are important in bacterial-induced disease. In fact, there were moderate (but significant) increases in surface expression of IL-7Rα on splenic macrophage and DC populations in Hb-infected-Rag2
−/− mice. Multiple myeloid cell populations, along with inflammatory cytokines and chemokines associated with their function, were decreased with anti-IL-7Rα antibody treatment in Hb-infected Rag2
−/− mice. Our findings differ from the conclusions reported by Shinohara and colleagues in which they used an adoptive transfer model of colitis to show that expression of IL-7Rα on CD4+ T cells, but not on other cells (NK cells, granulocytes, macrophages, and DC), was essential for development of colitis . However, the adoptive transfer model of colitis is dependent on IL-7R-mediated expansion of T cells for induction of colitis, in contrast to our models wherein bacterial antigens drive inflammation and colitis involving resident host T cells (when present) and other innate immune cells. It is possible that IL-7R expression by cells other than CD4+ T cells has a modest effect in their model as they noted a trend of increased colitis scores in IL
−/− x Rag2
−/− recipients of wildtype naïve T cells compared to Rag2
−/− -only recipients, although these differences were not significant .
Our studies using Mdr1a
−/− and Rag2
−/− mice strongly suggest that cells other than CD4+ T cells are important in bacterial-induced colitis. DCs along with other APCs, are likely involved in the development of colitis . In Hb-infected Rag2
−/− mice, we found that both pDCs and mDCs were significantly increased with colitis and dramatically decreased with anti-IL-7Rα treatment, especially IL-7Rα+ pDCs. In Mdr1a
−/− mice, DC-derived cytokines and DC-activation markers CD80 and CD86 were increased with disease and significantly decreased with anti-IL-7Rα antibody treatment. These results are highly consistent with other studies in T-cell sufficient mice, where IL-7 production from DCs and/or IL-7R signaling in DCs may regulate the proliferation and activation status of CD4+ T cells  involved in colitis.
Our results showed that NK cells may be associated with Hb-induced colitis. We found that lymphotactin, produced by and chemotactic for NK cells, was increased with disease and decreased following anti-IL-7Rα M595 treatment in both Rag2
−/− and Mdr1a
−/− mice infected with Hb. A regulatory role for NK cells was demonstrated in one adoptive transfer model of IBD , whereas another study showed NK cells neither suppressed nor exacerbated adoptive transfer-induced IBD . Our findings using Rag2
−/− mice infected with Hb indicated that total NK and IL-7Rα+ NK cells were associated with promoting IBD rather than suppression. Further studies using various IBD models are needed to clarify the function of NK cells in experimental colitis.
IL-7 promotes inflammation in part via activation of monocytes and macrophages  and induces proinflammatory cytokines and chemokines. In our two models, blockade of IL-7Rα decreased colonic F4/80+ cells and CD68 mRNA in Hb-infected Mdr1a
−/− mice, decreased subpopulations of macrophages in Hb-infected Rag2
−/− mice, and reduced monocyte-derived chemokines in both Hb-infected Mdr1a
−/− and Rag2
−/− mice. Shinohara and colleagues report that only IL-7R+CD4+ T cells (and not NK cells, granulocytes, DCs, or macrophages) contribute to colitis in their adoptive transfer model , whereas von Freeden-Jeffry and colleagues reported that in the absence of T and B cells, IL-7 dramatically increases F4/80+ cell infiltration into the intestinal mucosa upon H. hepaticus infection resulting in chronic intestinal inflammation . The main difference between these studies is the use of adoptive transfer of T cells into a lymphopenic environment vs. use of a bacterium to trigger IBD. Whereas T cells are important for IBD pathogenesis, our conclusions that non-T cells are also important in intestinal inflammation is aligned with von Freeden-Jeffry’s results. Our results using pharmacological blockade of IL-7Rα in T-cell sufficient and T- and B-cell-deficient mice clearly establish the contributions of both T cells and non-T cells such as macrophages, DCs and NK cells in IBD pathogenesis.