Natural killer cell levels in older adult mice are gender-dependent: thyroxin is a gender-independent natural killer cell stimulant.

The present study aimed to quantitatively analyze the effects of thyroxin on natural killer (NK) cells in the bone marrow and spleen of older adult (6 months) mice of both sexes. Five intraperitoneal injections of thyroxin were administered over 10 days in an attempt to stimulate the virtually negligible levels of NK cells in these older mice. Immunoperoxidase labeling of an NK cell surface marker, together with a tetrachrome hematologic counterstain, permitted morphological microscopic identification of mature NK cells, distinct from all other hemopoietic and immune (lymphoid) cells in the spleen and bone marrow. The results revealed that in spite of similar total cell content in both the spleen and bone marrow in both sexes at this age, there were significant, gender-based differences in NK cells and various other hemopoietic and immune cells. NK cells in the 6-month-old female mice are significantly more abundant than those of males at this age. Granulocytes in females were also significantly more numerous in both the spleen and bone marrow than in males. By contrast, monocytes and nucleated erythroid cells were significantly more numerous in males of this age. In both sexes, at this postbreeding age, the immunostimulant, thyroxin, had the common effect of significantly elevating the levels of NK cells. Our results underline the need for considering the two basic parameters of age and gender, as potentially confounding variables, prior to therapeutic administration of thyroxin, and possibly a host of hormones, cytokines and other regulators of hemopoietic and immune phenomena.