Regulatory T cells (Tregs) fulfill a central role in immune regulation. We reported previously that the integrin α_Eβ₇ discriminates distinct subsets of murine CD4⁺ regulatory T cells. Use of this marker has now helped to unravel a fundamental dichotomy among regulatory T cells. α_E⁻CD25⁺ cells expressed L-selectin and CCR7, enabling recirculation through lymphoid tissues. In contrast, α_E-positive subsets (CD25⁺ and CD25⁻) displayed an effector/memory phenotype expressing high levels of E/P-selectin–binding ligands, multiple adhesion molecules as well as receptors for inflammatory chemokines, allowing efficient migration into inflamed sites. Accordingly, α_E-expressing cells were found to be the most potent suppressors of inflammatory processes in disease models such as antigen-induced arthritis.

The use of monoclonal antibodies (mAbs) as therapeutic tools has increased dramatically in the last decade and is now one of the mainstream strategies to treat cancer. Nonetheless, it is still not completely understood how mAbs mediate tumor cell elimination or the effector cells that are involved. Using intravital microscopy, we found that antibody-dependent phagocytosis (ADPh) by macrophages is a prominent mechanism for removal of tumor cells from the circulation in a murine tumor cell opsonization model. Tumor cells were rapidly recognized and arrested by liver macrophages (Kupffer cells). In the absence of mAbs, Kupffer cells sampled tumor cells; however, this sampling was not sufficient for elimination. By contrast, antitumor mAb treatment resulted in rapid phagocytosis of tumor cells by Kupffer cells that was dependent on the high-affinity IgG-binding Fc receptor (FcγRI) and the low-affinity IgG-binding Fc receptor (FcγRIV). Uptake and intracellular degradation were independent of reactive oxygen or nitrogen species production. Importantly, ADPh prevented the development of liver metastases. Tumor cell capture and therapeutic efficacy were lost after Kupffer cell depletion. Our data indicate that macrophages play a prominent role in mAb-mediated eradication of tumor cells. These findings may help to optimize mAb therapeutic strategies for patients with cancer by helping us to aim to enhance macrophage recruitment and activity.

Regulatory T cells (Tregs) play a fundamental role in the suppression of different immune responses; however, compartments at which they exert suppressive functions in vivo are unknown. Although many groups have described the presence of Tregs within inflammatory sites, it has not been shown that inflamed tissues are, indeed, the sites of active suppression of ongoing immune reactions. Here, by using α_E⁺ effector/memory-like Tregs from fucosyltransferase VII-deficient animals, which lack E/P-selectin ligands and fail to migrate into inflamed sites, we analyzed the functional importance of appropriate Treg localization for in vivo suppressive capacity in an inflammation model. Lack of suppression by Tregs deficient in E/P-selectin ligands demonstrates that immigration into inflamed sites is a prerequisite for the resolution of inflammatory reactions in vivo because these selectin ligands merely regulate entry into inflamed tissues. In contrast, control of proliferation of naive CD4⁺ T cells during the induction phase of the immune response is more efficiently exerted by the naive-like α_E^–CD25⁺ Treg subset preferentially recirculating through lymph nodes when compared with its inflammation-seeking counterpart. Together, these findings provide the first conclusive evidence that appropriate localization is crucial for in vivo activity of Tregs and might have significant implications for anti-inflammatory therapies targeting recruitment mechanisms.

Compelling evidence suggests that Foxp3⁺CD25⁺CD4⁺ Treg play a fundamental role in immunoregulation. We have previously demonstrated that Treg have to enter peripheral tissues to suppress ongoing inflammation. However, relatively little is known about how Treg acquire the expression of homing receptors required for tissue‐ or inflammation‐specific migration. Migratory properties of conventional naïve T cells are shaped by the tissue microenvironment and organ‐specific dendritic cells during priming. Here, we show that this basic concept also holds true for CD25⁺CD4⁺ Treg: Priming of Treg within peripheral LN led to the expression of selectin ligands, which facilitate migration into inflamed skin, whereas activation within mesenteric LN led to induction of the integrin α₄β₇, which is required for migration into mucosal tissues. Furthermore, we could establish in vitro culture systems containing either dendritic cells from mesenteric and peripheral LN, or retinoic acid and IL‐12 as polarizing compounds to induce mucosa‐ and skin‐seeking Treg, respectively. Together, our results demonstrate that Treg, similarly to conventional T cells, can be configured with organ‐selective homing properties allowing efficient targeting into distinct tissues.

Foxp3⁺CD25⁺CD4⁺ Treg play a fundamental role in the maintenance of self tolerance and the control of inflammatory reactions. Previous data demonstrated a division of labor between naive‐ and effector/memory‐like Treg subsets, which is largely based on their lymph node‐recirculating and inflammation‐seeking migration behavior, respectively. The chemokine receptor CCR7 is expressed on both types of Treg subsets, albeit at different levels. Whether it fulfills similar or distinct roles in these subsets has not been studied so far. We here show that the recirculation of naive‐like Treg through LN and, to some extent, the gut is dependent on CCR7. Lack of CCR7 not only prevents recirculation, but also almost completely abolishes the ability of naive‐like Treg to control the priming phase of an immune response. In contrast, CCR7 deficiency in effector/memory‐like Treg promotes their accumulation in inflamed sites, compatible with a role of CCR7 for exit from the tissue. Local Treg accumulation was accompanied by an enhanced suppression of inflammation. Together, our findings provide conclusive evidence that CCR7 expression on Treg differentially controls in vivo function of the naive‐ and effector/memory‐like subsets.

It is now generally accepted that CD4⁺CD25⁺ T_reg cells play a major role in the prevention of autoimmunity and pathological immune responses. Their involvement in the pathogenesis of chronic arthritis is controversial, however, and so we examined their role in experimental antigen-induced arthritis in mice. Depletion of CD25-expressing cells in immunized animals before arthritis induction led to increased cellular and humoral immune responses to the inducing antigen (methylated bovine serum albumin; mBSA) and autoantigens, and to an exacerbation of arthritis, as indicated by clinical (knee joint swelling) and histological scores. Transfer of CD4⁺CD25⁺ cells into immunized mice at the time of induction of antigen-induced arthritis decreased the severity of disease but was not able to cure established arthritis. No significant changes in mBSA-specific immune responses were detected. In vivo migration studies showed a preferential accumulation of CD4⁺CD25⁺ cells in the inflamed joint as compared with CD4⁺CD25^- cells. These data imply a significant role for CD4⁺CD25⁺ T_reg cells in the control of chronic arthritis. However, transferred T_reg cells appear to be unable to counteract established acute or chronic inflammation. This is of considerable importance for the timing of T_reg cell transfer in potential therapeutic applications.