Destined for the intestine: thymic selection of TCRαβ CD8αα intestinal intraepithelial lymphocytes

The immune system is composed of a variety of different T-cell lineages distributed through both secondary lymphoid tissue and non-lymphoid tissue. The intestinal epithelium is a critical barrier surface that contains numerous intraepithelial lymphocytes that aid in maintaining homeostasis at that barrier. This review focuses on T-cell receptor αβ (TCRαβ) CD8αα intraepithelial lymphocytes, and how recent advances in the field clarify how this unique T-cell subset is selected, matures, and functions in the intestines. We consider how the available evidence reveals a story of ontogeny starting from agonist selection of T cells in the thymus and finishing through the specific signaling environment of the intestinal epithelium. We conclude with how this story raises further key questions about the development of different ontogenic waves of TCRαβ CD8αα IEL and their importance for intestinal epithelial homeostasis.

IL-15 Promotes Survival But Not Effector Function Differentiation of CD8+ TCRαβ+ Intestinal Intraepithelial Lymphocytes

CD8 single-positive cells, including CD8αα+ and CD8αβ+ subsets, constitute the majority of TCRαβ+ intestinal intraepithelial lymphocytes (αβ iIEL) in mice. CD8+ αβ iIEL show significantly weaker responses to TCR stimulation in the presence of exogenous IL-2 than do CD8+ T cells of the central immune system. IL-15 is a T cell growth factor likely expressed in the intestine mucosa. To understand the role of IL-15 in CD8+ αβ iIEL biology, we compared the effects of exogenous IL-15 and IL-2 on the survival and primary responses of the two CD8+ αβ iIEL subsets in vitro. In contrast to the death of ∼60% of both CD8αα+ and CD8αβ+ iIEL cultured in IL-2 with or without TCR stimulation, IL-15 promoted survival of the CD8αα+ subset in the presence of TCR stimulation and promoted survival of both subsets in the absence of TCR stimulation. The higher proliferation level of TCR stimulated CD8αα+ αβ iIEL cultured in IL-15 compared with those cultured in IL-2 is likely due to IL-15’s prosurvival effects. In addition, unlike exogenous IL-2, exogenous IL-15 did not support the effector functions of either iIEL subsets, including IFN-γ production, IL-4-induced Th2 cytokine production, and anti-TCR mAb-redirected cytotoxicity. These findings demonstrate that IL-15 and IL-2 are functionally distinct and suggest that IL-15 plays a unique role in the maintenance of the CD8+ αβ iIEL pool in the absence of Ag stimulation and in the survival and expansion of CD8αα+ αβ iIEL upon Ag stimulation.

Extrathymic T Cell Differentiation in the Human Intestine Early in Life

It is clear from experimental studies in mice that T cell maturation can occur outside the thymus, especially in the intestine. There is little sound evidence so far that extrathymic T cell maturation occurs to any significant extent in human gut, and, postnatally, there is abundant evidence that the gut mucosa is an immune effector organ. Here, we describe a large population of T lymphocytes in human fetal intestinal mucosa that are proliferating (Ki67+) in the absence of foreign Ag (CD3+, Ki67+ lamina propria lymphocytes (LPL) 22 ± 1.8% and CD3+, Ki67+ intraepithelial lymphocytes (IEL) 9.1 ± 1.4%), that express the T cell activation markers CD103, HLA-DR, and L-selectinlow, and that express mRNA transcripts for pre-TCR-α. There is also a substantial proportion of CD7+ LPLs that do not express CD3 (CD3−7+, 14 ± 7% of all LPLs) in the fetal gut that may be differentiating into CD3+ cells. Rearranged TCR-β transcripts of fetal LPLs, IELs, and paired blood lymphocytes were cloned and sequenced, and virtually no overlap of clonality was observed between blood and intestine, suggesting that gut T cells may not be derived from the blood. In addition, 30 days after engraftment of SCID mice with fetal intestine, CD3−7+ cells, proliferating T cells, and pre-TCR-α transcripts were abundant, and there is a threefold increase in CD3+ IELs. These data show that in the human intestine before birth a population of precursor T cells exists that may be differentiating into mature T cells in situ

The Role of IL-7 in Thymic and Extrathymic Development of TCRγδ Cells

IL-7-deficient (IL-7−/−) mice have reduced numbers of B and TCRαβ cells, but lack mature TCRγδ cells. Although most T cell development occurs in the thymus, some intestinal intraepithelial lymphocytes (IEL), including TCRγδ cells, can develop extrathymically. Epithelial cells in both thymus and intestine synthesize IL-7, suggesting that TCRγδ cell development could occur in either site. To evaluate the role of thymic IL-7 in development of TCRγδ cells, newborn TCRβ-deficient (TCRβ−/−) thymi were grafted to IL-7−/− mice. Donor- and host-derived TCRγδ cells were recovered from thymus grafts, spleen, and IEL. However, when IL-7−/− thymi were grafted to TCRβ−/− mice, no development of graft-derived TCRγδ cells occurred, indicating that extrathymic IL-7 did not support TCRγδ IEL generation from newborn thymic precursors. In contrast, TCRγδ IEL development occurred efficiently in adult, thymectomized, irradiated C57BL/6J mice reconstituted with IL-7−/− bone marrow. This demonstrated that extrathymic development of TCRγδ IEL required extrathymic IL-7 production. Thus, intrathymic IL-7 was required for development of thymic TCRγδ cells, while peripheral IL-7 was sufficient for development of extrathymic TCRγδ IEL.

Increased Threshold for TCR-Mediated Signaling Controls Self Reactivity of Intraepithelial Lymphocytes

To examine the effect of self Ag on activation requirements of TCR-αβ intestinal intraepithelial lymphocytes (IELs), we utilized the 2C transgenic (Tg) mouse model specific for a peptide self Ag presented by class I MHC, H-2Ld. CD8αα and CD4−CD8− IELs from syngeneic (H-2b, self Ag−) and self Ag-bearing (H-2b/d, self Ag+) strains were examined for their ability to respond in vitro to P815 (H-2d) cell lines expressing the endogenous antigenic peptide, p2Ca. Proliferation, cytokine production, and CTL activity were elicited in IEL T cells isolated from self Ag− H-2b mice when stimulated with P815 cells expressing basal levels of self Ag. These responses were enhanced following the addition of exogenous p2Ca peptide and ectopic expression of the costimulatory molecule, B7-1. By comparison, IEL from self Ag-bearing mice failed to respond to basal levels of self Ag presented by P815 cells even in the presence of B7-1-mediated costimulation. However, the addition of increasing amounts of exogenous p2Ca peptide induced a response from the in vivo “tolerized” T cells. These results suggest that exposure to self Ag in vivo increased the threshold of TCR activation of Ag-exposed self-reactive IELs. The dependence of increased signal 1 to activate self-reactive IELs suggests a defect in TCR signaling that may maintain self tolerance in vivo. These data suggest that conditions that overcome signal 1 IEL defects may initiate autoreactive responses in the intestine.

Activation Requirements, Lytic Mechanism, and Development of a Novel Anti-CD8-Resistant CTL Population

Almost all conventional CD8+ CTL and their CD8+ precursors are inhibited by anti-CD8 mAb. This requirement for CD8 function reflects both an avidity-augmentation role and a signal-transduction role for CD8 on T cells. We have, however, previously identified and partially characterized a novel functional population of CD8+, but anti-CD8-resistant, MHC class I-allospecific CTL. These CTL have unusual activation requirements in that their efficient generation in vitro requires inhibition of the CD8 avidity contribution (but not the CD8 signaling contribution), by anti-CD8 mAb. In this study, we have investigated the relationship of anti-CD8-sensitive and anti-CD8-resistant CTL by several criteria. These CTL populations share the phenotypic markers we have tested to date, they have similar but not identical Ag-specific repertoires, and they both appear to be generated from naive unprimed T cells. However, anti-CD8-sensitive and anti-CD8-resistant CTL populations exhibit important functional differences. They differ in their kinetics of activation in vitro, their dependence on exogenous cytokines, their use of lytic effector mechanisms, and their tissue distribution during ontogeny. Based on these results, we favor the hypothesis that these CTL populations represent distinct T cell lineages or subsets, and not merely different TCR avidity ranges within a single T cell lineage or subset.

T cell progenitors in the murine small intestine

Lymphocytes in the murine small intestine epithelium are known to have a high proportion of extrathymic T cells. To explore the possibility that small intestine intraepithelial lymphocytes (IELs) are derived from T cell progenitors present within the intestine, intestine-derived cells with characteristics of early-stage T cell precursors were studied for their ability to regenerate IEL T cell populations following transfer into irradiated recipient mice. Cells within this population lacked markers of mature T cells but expressed heat-stable antigen, the c-kit receptor for stem cell factor, and/or the pre-T cell alpha gene. Upon adoptive transfer, donor cells preferentially homed to the intestine and did not repopulate the thymus or extraintestinal peripheral lymphoid tissues. IELs derived from the donor precursor pool included both (alpha beta and gamma delta T subsets and consisted of phenotypically heterogeneous cell populations defined by CD4 and CD8. These findings provide evidence that T cell progenitors located in the intestinal mucosa are the likely source of most intestinal IELs.