Functional and Structural Aspects of the Epithelial Lymphocyte, with Implications for Coeliac Disease and Tropical Sprue

Abstract

Epithelial lymphocytes comprise a compartmentalised and specialised population of presumed effector cells which, in general, express the surface phenotypes (Lyt-2⁺; OX2⁺; OKT8⁺) of suppressor/cytolytic (T_s/c) cells. Granular cells within this population (gEL) morphologically resemble the circulating large granular lymphocytes (LGL) which subserve spontaneous (NK) cytolytic activity. Recent in vitro results indicate that gEL can develop this function after prolonged in vitro culture; the relevance of this, in vivo, remains to be decided. EL also appear to be able to mediate ADCC with sIgA against enteric micro-organisms. This is the kind of integrated activity that might be anticipated from local immunocytes within the intestinal mucosa. Other recent work suggests that gEL are not precursors of mucosal mast cells. EL also appear to be capable of inducing Ia-like expression in surface and crypt enterocytes, a property enjoyed both by highly purified T_h, but also T_S/c, cells as well. This raises the interesting prospect that enterocytes may display antigen in macrophage-like fashion to other adjacent cells within the inter-epithelial cell spaces. These latter observations might be more consistent with the presence of ‘activated’ and ‘blast-transformed lymphocytes in such conditions as coeliac disease and tropical sprue. Another emergent view that demands appropriate attention is that the infiltrate of T_s/c cells into surface, and crypt, epithelium of coeliac mucosa does not necessarily cause injury or damage to the jejunal tissues. Nevertheless the role, either primary or secondary, that EL play either in coeliac disease or tropical sprue still remains obscure.