β1 integrins are not required for the maintenance of lymphocytes within intestinal epithelia

Epithelial HVEM maintains intraepithelial T cell survival and contributes to host protection

Intraepithelial T cells (IETs) are in close contact with intestinal epithelial cells and the underlying basement membrane, and they detect invasive pathogens. How intestinal epithelial cells and basement membrane influence IET survival and function, at steady state or after infection, is unclear. The herpes virus entry mediator (HVEM), a member of the TNF receptor superfamily, is constitutively expressed by intestinal epithelial cells and is important for protection from pathogenic bacteria. Here, we showed that at steady-state LIGHT, an HVEM ligand, binding to epithelial HVEM promoted the survival of small intestine IETs. RNA-seq and addition of HVEM ligands to epithelial organoids indicated that HVEM increased epithelial synthesis of basement membrane proteins, including collagen IV, which bound to β₁ integrins expressed by IETs. Therefore, we proposed that IET survival depended on β₁ integrin binding to collagen IV and showed that β₁ integrin-collagen IV interactions supported IET survival in vitro. Moreover, the absence of β₁ integrin expression by T lymphocytes decreased TCR αβ⁺ IETs in vivo. Intravital microscopy showed that the patrolling movement of IETs was reduced without epithelial HVEM. As likely consequences of decreased number and movement, protective responses to Salmonella enterica were reduced in mice lacking either epithelial HVEM, HVEM ligands, or β₁ integrins. Therefore, IETs, at steady state and after infection, depended on HVEM expressed by epithelial cells for the synthesis of collagen IV by epithelial cells. Collagen IV engaged β₁ integrins on IETs that were important for their maintenance and for their protective function in mucosal immunity.

Immature hematopoietic cells display selective requirements for adhesion- and degranulation-promoting adaptor protein in development and homeostatsis

Adhesion- and degranulation-promoting adaptor protein (ADAP) modulates T cell development and function and promotes TCR signaling. Regulation of ADAP protein expression during thymopoiesis and in development of other hematopoietic lineages has not been explored. Using intracellular staining, we detected ADAP protein in bone marrow lymphocyte precursors. Like its binding partner SH2-containing leukocyte phosphoprotein of 76 kDa, ADAP is dynamically regulated during thymocyte positive selection. ADAP is also found in unconventional thymocytes, including NKT, CD8alphaalpha, and TCRgammadelta T cells. In peripheral T cells, ADAP is up-regulated after TCR stimulation and with acquisition of memory status. Although absent in splenic B cells, ADAP is present in pro-B cells, as well as in BM erythrocyte and myeloid progenitors. Studies with radiation chimeras show that ADAP is dispensable for NKT, CD8alphaalpha and TCRgammadelta T cell development, while confirming that ADAP is required for optimal development of conventional TCRalphabeta T cells in the thymus. Interestingly, ADAP is necessary for CD8alphaalpha homeostasis in the small intestinal epithelium, yet is dispensable for optimal reconstitution of splenic B cell populations. Our observations highlight the dynamic regulation of ADAP during T cell maturation and document expression patterns that suggest a possible role for ADAP in development of non-T hematopoietic lineages.

Intraepithelial lymphocytes: their shared and divergent immunological behaviors in the small and large intestine

At the front line of the body's immunological defense system, the gastrointestinal tract faces a large number of food-derived antigens, allergens, and nutrients, as well as commensal and pathogenic microorganisms. To maintain intestinal homeostasis, the gut immune system regulates two opposite immunological reactions: immune activation and quiescence. With their versatile immunological features, intraepithelial lymphocytes (IELs) play an important role in this regulation. IELs are mainly composed of T cells, but these T cells are immunologically distinct from peripheral T cells. Not only do IELs differ immunologically from peripheral T cells but they are also comprised of heterogeneous populations showing different phenotypes and immunological functions, as well as trafficking and developmental pathways. Though IELs in the small and large intestine share common features, they have also developed differences as they adjust to the two different environments. This review seeks to shed light on the immunological diversity of small and large intestinal IELs.

Generation of gut-homing T cells and their localization to the small intestinal mucosa

The intestinal mucosa represents the largest body surface toward the external environment and harbors numerous T lymphocytes that take up resident within the intestinal epithelium or in the underlying lamina propria (LP). The intraepithelial lymphocytes include subsets of 'unconventional' T cells with unclear ontogeny and reactivity that localize to this site independently of antigen-specific activation in secondary lymphoid organs. In contrast, the majority of the 'conventional' gut T cells are recruited into the intestinal mucosa subsequent to their activation in intestinal inductive sites, including Peyer's patches (PPs) and mesenteric lymph nodes (MLNs). T cells homing to the small intestine express a distinct pattern of homing molecules, allowing them to interact with and transmigrate across intestinal postcapillary endothelium. At least some of these homing molecules, including the integrin alpha(4)beta(7) and the chemokine receptor CCR9, are induced on T cells during their activation in PPs or MLNs. Mucosal dendritic cells (DCs) play a key role in this process, but not all intestinal DCs possess the ability to confer a gut-homing capacity to T cells. Instead, functionally specialized CD103(+) DCs derived from the small intestinal LP appear to selectively regulate T-cell homing to the small intestine.