Origins of intestinal intraepithelial lymphocytes: direct evidence for a thymus-derived gamma delta T cell component

GPR18 Controls Reconstitution of Mouse Small Intestine Intraepithelial Lymphocytes following Bone Marrow Transplantation

Specific G protein coupled receptors (GPRs) regulate the proper positioning, function, and development of immune lineage subsets. Here, we demonstrate that GPR18 regulates the reconstitution of intraepithelial lymphocytes (IELs) of the small intestine following bone marrow transplantation. Through analysis of transcriptional microarray data, we find that GPR18 is highly expressed in IELs, lymphoid progenitors, and mature follicular B cells. To establish the physiological role of this largely uncharacterized GPR, we generated Gpr18^-/- mice. Despite high levels of GPR18 expression in specific hematopoietic progenitors, Gpr18^-/- mice have no defects in lymphopoiesis or myelopoiesis. Moreover, antibody responses following immunization with hapten-protein conjugates or infection with West Nile virus are normal in Gpr18^-/- mice. Steady-state numbers of IELs are also normal in Gpr18^-/- mice. However, competitive bone marrow reconstitution experiments demonstrate that GPR18 is cell-intrinsically required for the optimal restoration of small intestine TCRγδ⁺ and TCRαβ⁺ CD8αα⁺ IELs. In contrast, GPR18 is dispensable for the reconstitution of large intestine IELs. Moreover, Gpr18^-/- bone marrow reconstitutes small intestine IELs similarly to controls in athymic recipients. Gpr18^-/- chimeras show no changes in susceptibility to intestinal insults such as Citrobacter rodentium infections or graft versus host disease. These data reveal highly specific requirements for GPR18 in the development and reconstitution of thymus-derived intestinal IEL subsets in the steady-state and after bone marrow transplantation.

ICOS promotes IL-17 synthesis in colonic intraepithelial lymphocytes in IL-10-/- mice

In the absence of IL-10, colonic inflammation ensues, which is characterized by high levels of IL-17. Here, we demonstrate a direct correlation between ICOS expression and IL-17 production in cIELs. IL-10(-/-) mice had increased numbers of cIELs and greater colon weight. Although the CD69 early activation antigen was expressed on cIELs from normal and IL-10(-/-) mice, ICOS was expressed only on cIELs from IL-10(-/-) mice. IL-17-producing cells in IL-10(-/-) mice consisted of CD4(+) and CD8(+) cIELs; however, CD4(+) cells were the predominant IL-17-producing cell population. Culture of cIELs from IL-10(-/-) mice with IL-23 resulted in an increase in ICOS and IL-17 expression, whereas IL-10 suppressed expression of ICOS and IL-17. This occurred in primary cultures and recall stimulation experiments. The ICOS ligand B7RP-1 was up-regulated on colonic epithelial cells and on a population of large granular leukocytes during inflammation. Culture of cIELs with B7RP-1(+) DCs enhanced IL-17A production from normal cIELs but failed to do so using cIELs from ICOS(-/-) mice. In vivo treatment of IL-10(-/-) mice with antibody to ICOS resulted in a significant reduction in colonic pathology. These findings implicate ICOS as an activational signal of Th17 cells during chronic intestinal inflammation, and they suggest that under some conditions, control of ICOS expression may help to suppress chronic intestinal inflammation.

Thymic differentiation of TCRαβ+CD8αα+IELs

Intraepithelial lymphocytes (IELs) contain several subsets, but the origin of the T-cell receptor (TCR)alphabeta(+) CD8 alpha alpha(+) IELs has been particularly controversial. Here we provide a synthesis, based on recent work, that attempts to unify the divergent views. The intestine has a primordial function in lymphopoiesis, and precursors with the potential to differentiate into T cells are found both in the epithelium and underlying lamina propria. Moreover, the thymus has been reported to export cells to the intestine that are not fully differentiated. TCR alpha beta(+) CD8 alpha alpha(+) IELs can differentiate in the intestine from each of these sources, but in normal euthymic mice, the thymus appears to be the major source for TCR alpha beta(+) CD8 alpha alpha(+) IELs. This unique IEL subset is a self-reactive population that requires exposure to self-agonists for selection in the thymus, similar to other regulatory T-cell populations. IELs transition through a double-positive (DP) intermediate in the thymus, but they originate from a subset of the DP cells that can be identified by its expression of CD8 alpha alpha homodimers. The agonist-selected cells in the thymus are TCRbeta(+) but CD4 and CD8 double negative. The evidence suggests that reacquired expression of CD8 alpha alpha and downregulation of CD5 occur after thymus export, perhaps in the intestine under the influence of interleukin-15. As a result of agonist exposure, a new gene expression program is activated. Therefore, the increased understanding of the developmental origin of TCR alpha beta(+) CD8 alpha alpha(+) IELs may help us to understand how they participate in immune regulation and protection in the intestine.

Significance of unconventional peripheral CD4+CD8dim T cell subsets

Routine T cells phenotyping occasionally reveals a CD4+CD8dim T cell subset with an apparently homogeneous dot plot. The aim of this study was to elucidate their immunological significance from analysis of 31 healthy donors, 21 elderly and 220 immune deficient patients. CD4+CD8dim T cells expressed reduced levels of CD8 (11-17,000 compared to 96-128,000 mol/cell on CD8+ T Cells). CD4 was expressed at the same level as on CD4+ T cells. The occurrence of raised CD4+CD8dim T cells (> 20 cells/muL) was similar in kidney transplant recipients (28.4%) and healthy donors (26%). It was somewhat lower in HIV+ patients (19.7%) possibly due to virally induced CD4+ T lymphopenia. However, an age effect is possible because the occurrence was raised (33.3%) in 70 volunteers (chi2 test NS). On the other hand, the size of the CD4+CD8dim subset was not correlated with age. CD4+CD8dim T cells did not express the activation markers CD69 (n = 220) or CD25 (n = 10) and expressed the homodimeric (alphaalpha) isoform of CD8, suggesting they are related to mucosal immunity (MALT). We selected 29 patients with unambiguous dot plots. In 26 of them one predominant TCR Vbeta clonotype was expressed on 18 to 94% of CD4+CD8dim T cells and never on more than 10% of conventional T cells. The predominant clonotypes were Vbeta8 (n = 5), Vbeta2 (n = 4), Vbeta13.1 and Vbeta 21 (n = 3 each). Whether this reveals a chronic stimulation or an emerging lymphoproliferative disorder must be elucidated. We propose to name this entity: "Oligoclonal Clonopathy of Undetermined Significance (OCUS)."

CD3 bright lymphocyte population reveal ?? T cells

BACKGROUND

In routine CD3/CD4/CD8 T-cell analysis, a CD3 bright population of lymphocytes is frequently observed. The aim of the present study was to identify the immunological significance of such CD3 bright lymphocytes.

METHODS

We analyzed samples from 31 healthy adult volunteers, 78 human immunodeficiency virus (HIV)-positive, and 78 renal transplanted patients.

RESULTS

A clearly distinct CD3 bright (frequently CD4-/CD8-) T-cell fraction was observed in 84% of donors and was directly correlated with the fraction of gammadelta T cells (r2 = 0.64). CD3 overexpression on gammadelta T cells was confirmed by a combination of monoclonal antibody staining (CD3-ECD, gammadeltaTCR-FITC, and alphabetaTCR-PE-Cy5) or immunomagnetic purification of gammadelta T cells (i.e., MdFI 20 vs 8.86). The gammadelta T cells expressed CD8 polypeptide chains (alpha and beta) in all possible combinations. The largest proportion, surprisingly, were cells expressing CD8betabeta homodimers (43.8 +/- 16.5%). CD8alphaalpha homodimers were expressed on 14.2% (+/- 12.3) of total gammadelta T cells, whereas CD8alphabeta heterodimers were expressed on 12.2% (+/- 7.5). We also observed a bimodal distribution of the intensity of CD3 fluorescence of gammadelta T cells in immunocompromised patients with a threshold at 105 cell/microl. CD3 bright gammadelta T cells were more frequently observed in HIV patients (29%) compared with renal transplant patients (11%) and healthy donors (3%; chi2 test: P = 0.0007).

CONCLUSIONS

The simple observation of a CD3 bright T-cell subset on CD3/CD4/CD8 routine analysis suggests a high gammadelta T-cell fraction and, in our opinion, should be followed by a complementary analysis to determine precisely the number of gammadelta T cells and to identify their CD8alpha/beta phenotype. When CD3 bright T cells/microl were more than 40%, high gammadelta T cells were detected in more than 87% of cases, with a specificity of 76%. Occasionally, the CD3 bright subset appeared to be strongly homogeneous, suggesting an oligoclonal proliferation that could possibly reveal a chronic localized stimulation or an early lymphoproliferative disorder. Because the gammadelta T cells have interesting immunological peculiarities, the clinical significance of their quantitative abnormality should be clarified in diseases such as HIV, organ transplantation, autoimmunity and lymphoma.