The Ability of Neonatal Mice to Develop Immunity to Mycobacterium tuberculosis Shows Sex Differences, with Females Displaying Evidence of an Enhanced Immune Response.

Using four core genotypes (FCG) mice, we have previously shown a larger number of CD4⁺ and CD8⁺ T cells in the spleens of female mice, a sex difference that develops by postnatal day 7 and is retained through adulthood. This difference in splenic T cell number is a consequence of reduced thymic egress and reduced splenic seeding in male mice, caused in part by the male-specific perinatal surge of testosterone, and in part by Sry, which is overexpressed in this model. Here, we used the background strain for FCG mice (C57BL/6J) to ask whether sex influenced actual immunity in the postnatal period. Pups were immunized on postpartum days 1 or 3 with Mycobacterium tuberculosis (Mtb), challenged on day 7 with Mtb purified protein derivative (PPD), and sacrificed on day 8. Subsequent ex vivo challenges of splenocytes showed PPD-stimulated CD8⁺ responses (increased CD8⁺, increased CD8⁺CD44^hi, decreased CD8⁺CD44^hiCD127^-/lo) but no differences between males and females. However, when CD8⁺ T cells were analyzed for IFN-γ and IL-2 production, although there was no sex difference in mono-functional IFN-γ⁺ (100%) or IL-2⁺ (67%), only females (0% of males and 42% of females) produced bi-functional (IFN-γ⁺IL-2⁺) cells. Ex vivo PPD-stimulated responses of other relevant cells from the spleen showed no sex differences in dendritic cells (CD11c⁺CD86⁺IL-6⁺) but females had more (3-fold) IL-6-producing macrophages (F4/80⁺CD86⁺IL-6⁺) and reduced T regulatory cells (CD4⁺CD25⁺Foxp3⁺). We conclude that some sex differences in immunity are evident at one week of age in Mtb immunized mouse pups, with females exhibiting qualitatively superior Mtb-specific immune responses.