High MHC class I expression correlates with slow growth in UV-induced skincarcinomas in hairless mice

An experiment was set up to investigate the relationship if any, between ce ll surface MHC class I expression and the growth rate for skin tumors induc ed by two different UV radiation regimens in hairless mice. Two groups of 2 0 hairless mice were each irradiated with either a UVA radiation source (2 SED per session) or broad-spectrum UV radiation (UVB) (8.1 SED per session) 5 days a week during the entire experiment. In the WA group, 17 out of 20 animals developed tumors, and 10 of these grew to a diameter of greater tha n or equal to 5 mm. In the WE group, 19 out of 20 animals developed tumors, and 15 of these grew to a diameter of greater than or equal to 5 mm. The t umor induction time, i.e. the time from the start of UV treatment to tumor appearance, was found to be significantly longer (p<0.01) in the WA than in the UVB group. This is in accordance with previous findings. Of the 25 tum ors growing to a diameter of greater than or equal to 5 mm, 11 were establi shed as cultured cell lines (4 UVA and 7 UVB tumors). These uncloned cell l ines were analyzed for surface expression of major histocompatibility compl ex class I by FAGS analysis. There was a clear correlation between high MHC class I expression and slow growth of the individual tumors (p<0.05). This suggests a role for the MHC class I governed, i.e. cytotoxic T-cell-mediat ed, reactions in deciding the fate of UV-induced skin cancers. No correlati on was found between MHC class I expression and tumor induction time.