The role of the immune system in the pathogenesis of psoriasis

Psoriatic involved skin contains an increased number of activated T cells. The mechanism through which these T cells achieve and maintain their activated state is unknown, and both antigen-dependent and -independent mechanisms may contribute. Recently a novel pathway of antigen-independent T-cell activation has been described. This pathway is identified by a monoclonal antibody that binds to a T-cell membrane surface molecule termed "UM4D4.". This molecule is expressed on a minority (20%) of psoriatic peripheral blood T cells but on a majority (75%) of the T cells in lesional skin. Thus, UM4D4 could play a role in antigen-independent T-cell activation in psoriasis. Indeed the monoclonal antibody anti-UM4D4 consistently induces proliferation of psoriatic UM4D4+ T-cell clones. The activity of antigen-dependent pathways are also enhanced in psoriatic epidermis in as much as involved skin relative to uninvolved skin contains an increased number and function of antigen-presenting cells. Upon activation, the lesional T cells release lymphokines. Central to the immune hypothesis of psoriasis is that some of these T-cell lymphokines act on keratinocytes to induce changes characteristic of psoriasis. Indeed lymphokines from lesional psoriatic T-cell clones directly alter in vitro keratinocyte phenotype through induction of intercellular adhesion molecule-1 (ICAM-1) and HLA-DR cell-surface expression. Furthermore, the lymphokines also enhance keratinocyte growth. These data suggest a critical role for the immune system in the pathogenesis of psoriasis.