INTENSITY OF CLASS-I ANTIGEN EXPRESSION ON HUMAN TUMOR-CELL LINES ANDITS RELEVANCE TO THE EFFICIENCY OF NON-MHC-RESTRICTED KILLING

A modified tetrazolium reduction assay (MTT) was used to assess the re lation between HLA class I antigen expression on tumour cells and thei r susceptibility as a target for non-MHC restricted LAK/NK cytotoxicit y using interleukin-2 activated peripheral blood mononuclear cells (MN C) from normal individuals. At 20/1 effector/target ratio this ranged from no killing to 77%. The efficiency of killing was dependent on dur ation of effector cell culture with IL-2, peaking at day 10 and declin ing thereafter. This killing could be enhanced by addition of other cy tokines including interferons alpha, beta and gamma. Study of a panel of 15 tumour cell lines using a single effector showed that there was no statistically significant inverse correlation (using Spearman rank test) between the degree of tumour class I expression and LAK/NK killi ng at 20/1 (r = 0.23 P = 0.39) and 10/1 (r = 0.30, P = 0.27) and at 5/ 1 E/T ratio r = 0.47, P = 0.08) respectively. Lack of inverse correlat ion between these two parameters came from study of one bladder tumour line (FEN), whose absent class I antigens had been corrected by trans fection with beta2 microglobulin gene. At high E/T ratio (20/1) there was an increase in the susceptibility of target cells to lysis (36% pa rent cell, 45% transfected cell), whilst at lower E/T ratios (1/1) the re was significantly more killing of the non-transfected cells (10% vs 31%). The addition of anti-class I antibody W6/32 increased killing b y 18% but this was non-specific as the same increase occurred with a c lass II antibody. These data suggest that overall there was not an inv erse correlation between class I expression and LAK/NK killing at high E/T ratios, whilst at low (5/1 or lower) E/T ratios this correlation nearly reached statistical significance suggesting that the conflicing literature reports may be due to a threshold levels of effector cells above which the masking effects of MHC antigens disappears.