Skin-associated lymphoid tissue in human immunodeficiency virus-1, human papillomavirus, and herpes simplex virus infections.

The skin-associated lymphoid tissue is composed of keratinocytes, Langerhans cells, skin trophic T cells, and lymphatic endothelial cells of the skin. The epidermis, which is involved in many viral infections, contains all of the components needed for an effective immune response: antigen-presenting Langerhans cells, T cells, and cytokines from leukocytes and keratinocytes. There have been some recent advances in the study of the cutaneous immunology involved in infections with the human immunodeficiency virus (HIV), human papillomavirus (HPV), and herpes simplex virus (HSV). In general, viral diseases with cutaneous manifestations lead to a decline in epidermal Langerhans cell numbers, which probably reflects Langerhans cell emigration out of the epidermis and entry into regional lymph nodes, leading to Langerhans cell activation and antigen presentation to T cells. In HSV, there is a subsequent T-cell infiltration of the epidermis, composed of CD4+ cells that have both immune modulatory action and direct cytotoxic action. In HIV, where there is a systemic depletion of CD4+ cells, the epidermis is left with reduced numbers of T cells. Intradermal injection of interleukin-2, however, leads to an epidermal cellular infiltration in HIV+ individuals. In HPV-induced condyloma, intralesional interferon increases Langerhans cells and CD4+ and CD8+ cells in the skin, as well as transforming growth factor beta 1, tumor necrosis factor-alpha, pRB, and p53. Therefore, viral infections involving the epidermal immune system have certain similar characteristics, whereas other factors are unique to the infecting virus.