Adelheid Elbe

Dendritic epidermal T cells: Activation requirements and phenotypic characterization of proliferating cells

Dendritic epidermal T cells (DETC) are CD45+, Thy-1+, CD5-, CD8-, CD4- murine lymphocytes that express surface-bound CD3 antigens associated with T cell receptor gamma/delta heterodimers. Using epidermal cells greatly enriched for DETC and depleted of Langerhans cells, we found that DETC have growth requirements quite different from those of accessory cell-depleted lymph node and splenic T cells. Although the latter cells strongly proliferate in response to phorbol myristate acetate (PMA) + ionomycin, DETC, when exposed to interleukin-1 (IL-1), interleukin-3 (IL-3), concanavalin A (ConA), PMA, and ionomycin used either alone or in combination, do not exhibit significant mitotic activity. Recombinant interleukin 2 (rIL-2), albeit ineffective by itself, leads to vigorous proliferation of DETC when used with either ConA or PMA + ionomycin + IL-1. In contrast, the combination of PMA and recombinant interleukin-4 (rIL-4), which triggers growth of lymph node T cells, does not induce proliferation of DETC. Although a portion of proliferating DETC expressed CD8 antigens, essentially none bore detectable amounts of surface-bound CD4 or CD5 antigens, or both. Continuing stimulation of primary DETC cultures with lectin/lymphokine-rich media results in the propagation of cells with the essential phenotypic features of resident DETC.

Epidermal T lymphocytes — ontogeny, features and function

ConclusionsThe murine epidermis contains a network of Thy-1+ dendritic T cells. These T cells arise from early fetal stem cells and differentiate in the fetal or neonatal thymic or epidermal microenvironment. Their lack of expression of CD5, CD4, and CD8 antigens, as well as their virtually exclusive expression of a CD3/TCR Vγ3/Vδ1 complex, distinguishes DETC from the bulk of peripheral T cells.The early appearance of TCR γ/δ cells in ontogeny, the lack of expression of CD4 and CD8 antigens, and the relative paucity of γ and δ genes compared to α and β genes, indicates that γ/δ T cells provide a phylogenetically primitive, broadly acting, and poorly discriminating immunologic defense system. In this system, recognition of antigen is not restricted by classical MHC class I and class II antigens, but may occur in the context of relatively nonpolymorphic restricting elements, such as Qa [82], Tla [10] or CD1 [62]. This rather primitive immune system provided by DETC may serve to protect the epidermal integrity. Upon recognition of self proteins released following epidermal injury, DETC may become activated and assist in the removal of altered cells. In this limited fashion, the epidermis may be an independently competent immunologic system. However, the fact that the TCR repertoire of DETC does not allow for the recognition of antigenic peptides in conjunction with MHC moieties excludes the possibility that the diverse immune response elicited by topical contact with foreign antigens is mediated by DETC.Whether this statement also applies to the human epidermis cannot be answered at the present time. Let us consider a few plausible concepts concerning derivation and function of human epidermal T cells. First, one could postulate that in early ontogeny, the human epidermis harbors a small, indigenous population of naive T lymphocytes with monomorphic TCR representing an analogue to murine DETC. These cells could function in a manner similar to that proposed for murine DETC. They may even persist into adult life, so far undetected because they would be outnumbered by immigrating polymorphic T cells from peripheral lymphoid organs. Second, it is conceivable that the human epidermis contains an indigenous population of naive T lymphocytes with a polymorphic TCR repertoire representing a phylogenetically advanced analogue to murine DETC. Although equipped with TCR allowing antigen recognition in the context of MHC, their density is probably too low to make them an effective host defense system against the multitude of environmental antigens presented by Langerhans cells. One could rather assume that they proliferate upon recognition of self antigens occurring in a perturbed epidermis. The autoreactivity of these cells may not necessarily be beneficial. Finally, the fact that the entry of circulating HECA-452+ memory cells into the skin is dependent upon the injury-induced ELAM-1 expression by endothelial cells of the dermal microvasculature could indicate that all T cells present in adult human epidermis are recruited upon alteration of the skin. Following this reasoning, the human epidermis should not be regarded as a complete, self-sustaining immunologic organ but rather as a homing site for and a target of lymphocytes antigenically sensitized in peripheral lymphoid organs.

Lymphocyte Development and Immunoreactivity in IL-2 Deficient Mice

Ever since its first description, Interleukin-2 (IL-2) has been attributed a major role in the regulation of immune responses (for review see Smith 1988, 1992; Paul 1989). It is transiently produced by T cells after activation, predominantly but not exclusively by the CD4+ pre TH, THO and TH1 subtypes (Mosmann et al. 1989). IL-2 acts as a major autocrine and paracrine growth factor. However, it is not the only known T cell growth factor nor does it act exclusively on T cells. IL-2 promotes the growth and activity of NK cells (Trinchieri et al. 1984) and the induction of growth and differentiation of B cells (Zubler et al. 1984, Tigges et al. 1989) is a well recognized phenomenon. A role for IL-2 in intrathymic development has been suggested on the basis of the presence of IL-2 receptor bearing cells in the thymus and of inhibitory effects of anti IL-2 receptor antibodies on T cell development in vivo and in vitro (reviewed in Carding et al.1991). Other studies have, however, failed to observe such effects on the development of the major T cell subsets (Plum and de Smedt 1988). The very important role of IL-2 in vivo is also documented by the fact that three recognized cases of a congenital IL-2 defect in patients may lead to severe, and even lethal immunodeficiency (reviewed in Smith 1992).

The Role of Fetal Epithelial Tissues in the Maturation/Differentiation of Bone Marrow-Derived Precursors into Dendritic Epidermal T Cells (DETC) of the Mouse

Our attempts to clarify the contribution of the thymic vs. the cutaneous microenvironment in the maturation of dendritic epidermal T cell (DETC) precursors into DETC gave diverse results. In one series of experiments, we found that i.v. injection of fetal thymocytes (containing a TCR V gamma 3-expressing subpopulation), but not of adult thymocytes (containing no TCR V gamma 3+ cells) results in the appearance of CD3/TCR V gamma 3+ dendritic epidermal cells (=DETC). In other experiments, we have obtained evidence that transplantation of day 16 fetal skin onto a Thy-1-disparate recipient results in the appearance of donor-type DETC. Our further observation that the transplanted skin contains CD45+/Thy-1+/CD3- lymphocytes, but no mature T cells, therefore implies that fetal skin can provide stimuli promoting the expression of CD3/TCR genes in immature (CD3-) DETC precursors. It remains to be seen whether both or only one of these maturational pathways are (is) followed under physiological conditions.

Ontogeny, Features and Functions of Epidermal T Lymphocytes

In the past decade, it has become increasingly clear that the mammalian skin harbors an indigenous population of lymphocytes, almost all of which belong to the T cell system (Stingi et al., 1989). In man, the majority of cutaneous T lymphocytes are found in the dermis, where they are preferentially clustered around postcapillary venules and around the appendages (Bos et al., 1987). Intraepidermal T cells account for only ≤ 10% of all T cells within human skin. In mice, the epidermis contains a regular network of T cells (Tschachler et al., 1983). Because of their characteristic morphology, they are referred to as Thy-1+ dendritic epidermal cells (Thy-1+ DEC) or, more recently, as dendritic epidermal T cells (DETC; Stingi et al., 1989).

Differential Effects of Various Physicochemical Agents on Murine Ia- and Thy-1 Positive Dendritic Epidermal Cells

It is now well established tljat epidermal cells (EC) can initiate T-cell-dependent immune responses1and that this functional capacity can be altered by various physicochemical agents. Low-dose ultraviolet B (UV-B) irradiation of EC in vivo or in vitro impairs their sensitizing potential in contact hypersensitivity (CHS)2–4. Similarly, both man and experimental animals that had received psoralen plus UV-A (PUVA) treatment exhibited a markedly reduced CHS response to simple chemical haptens4–6. In the search for the mechanism(s) by which these various physicochemical agents modulate EC-induced immune responses, particular attention has been focused on the effects of these agents on Ia-bearing Langerhans cells (LC) because the immunostimulatory capacities of EC are critically dependent on these cells1,7–10. Results obtained showed that PUVA-treatment leads to a dramatic reduction of Ia-bearing dendritic EC as assessed by immunofluorescence (IF)11,12, and to concommitant impairment of LC-dependent immunologic functions of EC12. In the cage of low-dose UV-B, however, the functional defect of LC-containing EC8is reportedly not paralleled by a disappearance of surface Ia antigens3,11.