Estrogen influences class-switched memory B cell frequency only in humans with two X chromosomes.

Postpubertal cisgender females have higher levels of CD19+CD27+IgD- class-switched memory B cells compared with age-matched cisgender males.

• The difference was only observed after puberty and before menopause, suggesting a strong influence for sex hormones.
• No significant difference in class-switched memory B cell frequency was observed between prepubertal males and females.
• The study was conducted as a human-only ex vivo study comparing age-matched cisgender males and females across different life stages.
• Postmenopausal cisgender females showed reduced class-switched memory B cell frequencies compared with premenopausal females.

B cells express high levels of estrogen receptor 2 (ESR2), and class-switch-regulating genes are enriched for ESR2-binding sites.

• ESR2 was identified as the predominant estrogen receptor expressed on B cells.
• Bioinformatic analysis revealed that genes involved in class-switch recombination regulation are enriched for ESR2-binding sites.
• This finding provides a mechanistic basis for estrogen's influence on class-switched memory B cell frequency.
• ESR2 expression on B cells was noted as a key molecular link between estrogen signaling and B cell class switching.

Blockade of natal estrogen in transgender males (XX karyotype) reduced class-switched memory B cell frequency.

• Transgender males have an XX karyotype and received gender-affirming hormone therapy that included natal estrogen blockade.
• Reduction in class-switched memory B cell frequency following estrogen blockade supports a direct role for estrogen in maintaining elevated levels in XX individuals.
• This finding was observed in a gender-diverse cohort studied as part of the ex vivo human study.
• The result confirms that estrogen promotes class-switched memory B cell frequency specifically in individuals with an XX chromosomal background.

Gender-affirming estradiol treatment in transgender females (XY karyotype) did not increase class-switched memory B cell frequency.

• Transgender females have an XY karyotype and received exogenous estradiol as part of gender-affirming hormone therapy.
• Despite estradiol administration, class-switched memory B cell levels did not rise to levels seen in cisgender females.
• This finding indicates that estrogen alone is insufficient to increase class-switched memory B cell frequency without an XX chromosomal background.
• The result directly demonstrates a chromosomal requirement for estrogen's effect on this B cell subset.

Postmenopausal cisgender females taking hormone replacement therapy (HRT) had increased class-switched memory B cell frequencies compared with those not taking HRT.

• The comparison was made between postmenopausal cisgender females on HRT versus postmenopausal cisgender females not on HRT.
• Both groups share an XX karyotype, consistent with the finding that estrogen influences class-switched memory B cells only in XX individuals.
• This finding further supports the role of estrogen in maintaining elevated class-switched memory B cell frequencies in XX individuals across the lifespan.
• The HRT result provides a clinically relevant validation of the estrogen-XX chromosome interaction observed in younger cohorts.

Sex hormones and chromosomes work in tandem to impact immune responses, with estrogen only influencing class-switched memory B cell frequency in individuals with an XX chromosomal background.

• The study used a gender-diverse cohort to dissociate the effects of sex chromosomes from those of sex hormones.
• Evidence from transgender males (estrogen blockade reducing B cell frequency) and transgender females (estrogen addition not increasing B cell frequency) together establish the XX chromosomal requirement.
• The findings suggest that X-linked genes may interact with estrogen signaling pathways to regulate class-switch recombination.
• This represents a human-only ex vivo study, and the authors note the mechanism remains to be fully defined.