Sex-based immunological disparities, governed by an interplay of genetic determinants, hormonal and environmental influences, and microbiome composition, are consequential for a spectrum of health outcomes, particularly in the context of systemic autoimmune pathologies and immunogenic responses post vaccination.¹ Notably, females account for approximately 80% of the patient demographic with systemic autoimmune disorders and demonstrate markedly enhanced vaccine-induced immunogenicity relative to their male counterparts.² This heightened immunogenicity in females is attributed to more robust innate and adaptive immune mechanisms and superior pathogen recognition.^1,3

The intrinsic immunological differences driven by sexual dimorphism highlight the importance of ongoing investigations of their impact within immuno-oncology, particularly the use of immune checkpoint inhibitors (ICIs) in cancer treatment. Immune regulatory mechanisms, specifically CTLA-4 (cytotoxic T-lymphocyte—associated protein 4) and PD-1/PD-L1 (programmed death receptor—1/programmed death—ligand 1) pathways, are crucial for tumor-mediated immunosuppression. These pathways have been targeted to treat various advanced malignancies, including urothelial carcinoma.⁴ Similarly, the tumor microenvironment emerges as a vital factor in cancer progression and therapeutic outcomes. The sexually dimorphic pattern in immune infiltration, characterized by increased recruitment of dendritic cells, CD4+ T lymphocytes, and B lymphocytes in females, suggests enhanced initial immune detection and response. However, this immunological advantage is challenged by the emergence of sophisticated tumor resistance, including the upregulation of immune checkpoint molecules such as TIM-3, TIGIT, and VISTA, and the proliferation of myeloid-derived suppressor cells.⁵ Despite growing evidence of sex-based differences in survival rates among bladder cancer patients, there is still a significant gap in research exploring the interaction between patient sex and the effectiveness of new anticancer immunotherapies.

The observed biological differences raise critical questions about whether these discrepancies are simply numerical or indicative of deeper biological explanations. For instance, data presented in the Figure show that males receiving anti–PD-1/anti–PD-L1 therapies exhibit a lower risk of disease recurrence in primary metastatic/locally advanced urothelial cancer (CheckMate 901 male HR 0.72 [95% CI 0.57-0.90] vs female HR 0.68 [95% CI 0.45-1.03]) and in adjuvant immunotherapy settings (eg CheckMate 274 male HR 0.68 [95% CI 0.54-0.87] vs female HR 0.76 [95% CI 0.50-1.16]).^6,7 This pattern persists in nonmuscle-invasive bladder cancer treatments, as demonstrated by Keynote 057 and QUILT 3.032, which report lower complete response rates in females.^8,9 Despite the limitation of fewer female participants in these studies, which could cause a numerical bias in representation, one must consider why such differences are not as apparent in both neoadjuvant and adjuvant chemotherapy trials.^10-12 The consistency of these findings across various analyzed subgroups lends credibility to their significance, though the limited statistical power must be acknowledged.

Let’s consider potential reasons behind these sex differences. Variations in tumor mutational burden (TMB) could play a significant role. Tumors with high TMB typically allow for better T-cell–mediated identification and elimination of cancer cells. However, lower TMB levels observed in female patients could stem from more stringent immune selection driven by major histocompatibility complex class II during tumor development, along with a reduction in antigenic variability. This could lead to a weaker response to ICI therapies.^13,14 Furthermore, the rate of hypermutation in urothelial cancer tends to be higher in males (68%) compared to females (32%), highlighting a sex disparity in the genetic landscape of the tumor itself.¹⁵

Importantly, these differences in response to immunotherapy and cancer biology are not exclusive to urothelial carcinoma. Evidence from a meta-analysis of randomized clinical trials (RCTs) comparing the effectiveness of ICIs as monotherapy against traditional treatments devoid of immunotherapy indicates a consistently greater survival benefit for males over females in 19 of 20 RCTs, regardless of the cancer type, therapeutic line, or the particular ICI used (eg, anti–PD-1 or anti–CTLA-4). This discrepancy in survival advantage is statistically and clinically significant, with males experiencing a mortality risk reduction twice as large as that observed in females.¹⁶ This suggests a fundamental biological disparity in the effectiveness of immuno-oncology monotherapy, with outcomes that appear to be influenced by sex, irrespective of the tumor type.

Considering the nuanced differences in immune system behavior based on sex, women may experience greater benefits from immunotherapeutic strategies beyond the sole use of ICIs. For instance, the combination therapy of enfortumab vedotin and pembrolizumab, as detailed in the subgroup analysis by Powles et al during the 2024 American Society of Clinical Oncology Genitourinary Cancers Symposium, notably showed no significant differences in disease-free survival and overall survival between males and females.¹⁷

Regrettably, the significance of host factors, especially the sex of the patient, has not been adequately addressed in both preclinical studies and clinical trials concerning cancer immunotherapy. This oversight is evident in the disproportionately low enrollment of female participants in RCTs evaluating ICIs in urothelial cancer, among other cancer types.¹⁶ This sex bias is not confined to clinical studies, but extends to preclinical research, where the representation of male and female cell lines in resources like the Broad Institute’s Cancer Cell Line Encyclopedia is significantly skewed toward males for numerous cancer categories.¹⁸

Clinical investigations that predominantly enroll male participants and yield results that primarily benefit male cohorts might not be universally applicable to female populations, often leading to immunotherapeutic interventions tailored toward male efficacy and tolerability. The growing evidence for biological differences in immune responses in males and females underscores the necessity of equitable recruitment strategies in clinical trials to achieve sex-balanced research and therapeutic refinement, irrespective of disease prevalence.