CD8+ cytolytic T lymphocytes and the skin

T lymphocytes show a special affinity for the skin. Although the roles played by the CD4+ population of T lymphocytes in immunodermatology were so far actively investigated, much less is known about the roles played in the skin by CD8+ cytolytic T lymphocytes (CTL). The activity of CD8+ CTL in the immunodermatological context, however, is likely to be most important; the immuno-biology itself of CD8+ CTL, moreover, although far from being fully understood, shows intriguing characteristics. Immunophenotype, function and cytokine profile of CD8+ CTL are overviewed in the first section of this review. Phenotypically, not only CD8+ CTL can be subdivided into CD8+ CD28+ CD11b- and CD8+ CD28- CD11b+ subsets, but also an up-to-now undetected CD8+ CD28- CD11b- subset does exist. Functionally, not only "cytotoxic" but even "suppressor" subpopulations have been shown to exert cytolytic capabilities indeed, and "suppression" itself may be due to such a lytic capacity. According to cytokine synthesis, CD8+ CTL can be split into Tc1 and Tc2 subsets, each able to influence specific patterns of immune responses. The impact of CD8+ CTL in immunodermatology, overviewed in the second section of the current review, is crucial. The pathophysiology of inflammatory dermatoses is deeply influenced by the activity of CD8+ CTL: e.g., CD8+ CTL within psoriatic epidermis are possibly associated to the persistence of psoriatic lesions not undergoing resolution; on the other hand, in late lesions of lichen planus CD8+ CTL predominate, thus explaining presumably both the cytolytic attack against keratinocytes and the modulation of the inflammatory reaction up to the final resolution of the lesions, Tc1 cells are decreased in atopic dermatitis, and such a decrease can account both for IgE overproduction and for development of infections. Finally, CD8+ CTL can sustain against cutaneous viruses/tumors cytolytic immune responses not only of secondary but even of primary type, i.e. induced by Langerhans cells/dendritic cells either transfected or pulsed with skin virus/tumor-associated antigens, thus allowing the production of vaccines against cutaneous viral/neoplastic diseases.