Diploid expression of human leukocyte antigen class I and class II molecules on spermatozoa and their cyclic inverse correlation with inhibin concentration.

A diploid expression of class I and class II human leukocyte antigens (HLA) has been found in purified spermatozoa by using double fluorescence labeling cytofluorometry and relevant monoclonal antibodies; this expression has been confirmed for the first time by the analysis of specific HLA mRNA and metabolic 35S labeling followed by immunoprecipitation, which demonstrates an active ongoing translation of HLA proteins in germinal cells. Long-living mRNA coming from diploid germinal cells may be translated to HLA molecules in spermatozoa. This translation is controlled (or at least inversely correlated) by a testicular hormone (inhibin) in a cyclic fashion. Remarkably, serum levels of inhibin, synthesized by Leydig and Sertoli cells, follow a 12- to 13-day cycle, with a peak level at Day 6; this is probably controlled by FSH (not cyclic in males) and other testicular and/or unknown hormones. Peak levels of inhibin concur with the lower density and percentage of spermatozoa expressing both HLA class I and II molecules (close to 3% by cytofluorometry); lowest levels of inhibin coincide with the highest numbers (35-40%) of spermatozoa positive for both HLA molecules and a higher surface density. These observations could put to an end a disconcerting and long-lasting controversy on the expression/non-expression of HLA antigens on spermatozoa. The possibility that HLA-bearing spermatozoa are more capacitated for fertilization than those that do not bear HLA, and the implications of our results on male fertility control are also discussed.