Mechanisms of regression.

In this issue, Bayer-Garner et al.1 report on the immunopathology of regression in benign lichenoid keratosis, keratoacanthoma (KA), and halo nevus. They suggest that cytotoxic T-cells may be the common denominator of regression in these tumors. Other studies of halo nevi have shown abundant CD8+ T-cells and FXIIIa-positive histiocytes during regression, suggesting that the immune response is involved in tumor regression.2,3 Halo congenital nevi undergoing spontaneous regression also demonstrate infiltrating T-cells, with CD8+ cells outnumbering CD4+ cells. Natural killer cells are not numerous, but IgM antibodies against nevus cells and melanoma cells are noted.4 Humoral immunity may play a role in regression, or may simply represent an epiphenomenon. Cell-mediated immunity is consistently observed.