FEMALE CATS HAVE LOWER RATES OF APOPTOSIS IN PERIPHERAL-BLOOD LYMPHOCYTES THAN MALE CATS - CORRELATION WITH ESTRADIOL-17-BETA, BUT NOT WITHPROGESTERONE BLOOD-LEVELS

During earlier study, we quantified by flow cytometry the rate of apop totic feline lymphocytes after overnight culture. We found evidence th at the sex of the animals influences the rate of apoptosis, intact fem ales showed lower rates of apoptosis in lymphocytes cultured overnight than castrated male cats. This observation was also confirmed for cat s that were previously experimentally infected with the feline immunod eficiency virus (FIV). In an attempt to find an explanation for these sexually determined differences, plasma estradiol-17 beta and progeste rone levels were measured by radio-immune assay in the blood of these cats. The hormone levels were analyzed with respect to the rate of lym phocyte apoptosis. As expected, castrated males had lower blood levels of estradiol and progesterone than females. However, no overall corre lation was found between hormone blood levels and rate of apoptosis un der non-stimulating conditions. Interestingly, the rate of apoptosis f ound in lymphocytes collected from females and stimulated overnight in phytohaemaglutinin-containing medium, showed a strong negative correl ation with the estradiol levels in the blood of these cats. To our kno wledge, this is the first confirmation that estradiol in physiological concentrations may protect peripheral lymphocytes from apoptosis afte r stimulation. No correlation was found in male cats. In conclusion, t hese observations broaden the list of sexually determined differences of the immune system, sex and sex hormones predispose males and female s for certain immune responses and dysfunctions. The present observati ons have to be taken into account when designing or interpreting exper iments on apoptosis and, for example, evaluating the influence of a pr eexisting FIV infection on the rate of apoptosis. It would be highly a dvisable to include only spayed cats in studies on the immune system s o as to minimize the influence of sex hormones. (C) 1998 Elsevier Scie nce B.V. All rights reserved.