The protective role of Langerhans' cells and sunlight in multiple sclerosis

The low prevalence of multiple sclerosis in equatorial areas may be the con sequence of a physiological immunosuppression induced by ultraviolet radiat ion on the epidermal Langerhans' cells, which are the antigen-presenting ce lls. Multiple sclerosis appears to be the consequence of an immune response to an environmental antigen during childhood in the absence of two protect ive factors: a genetic factor conferred by the HLA haplotypes of Langerhans ' cells, and an environmental factor conferred by ultraviolet radiation in sunlight. In the presence of these two factors, T lymphocytes are physiolog ically immunosuppressed. Their association may play a key role in protectio n against multiple sclerosis. The varying combinations of these two factors may provide an explanation for the differences in incidence and prevalence of the disease noted within a single region or from one region to another throughout the globe. The hypothetical pathogenic mechanism developed in th is article opens up new fields of research, particularly those concerning i ts therapeutic applications. (C) 2000 Harcourt Publishers Ltd.