A CD74-high antigen-presenting myeloid state is associated with cardiac inflammation in immune checkpoint inhibitor-associated myocarditis

Background Myocarditis linked to immune checkpoint inhibitor therapy carries a substantial risk, and the myeloid immune events that drive it remain incompletely defined. In this study, we characterized the CD74-MHC-II-related myeloid inflammatory program in ICI-associated myocarditis and evaluated the effects of anti-CD74 antibody intervention in an anti-PD-1-induced mouse model. Methods We combined publicly available transcriptomic resources with human single-cell RNA sequencing data and, using cross-cohort concordance analysis, myeloid subset analysis, and module scoring, screened for immune programs associated with the disease. A murine myocarditis model driven by anti-PD-1 was subsequently built, and in vivo interventions were applied that comprised anti-CD74 antibody administration, IFN-γ neutralization, and JAK inhibition. Changes in myocardial injury and in the immune inflammatory response were then assessed by echocardiography, histopathology, immunofluorescence, Western blot, qPCR, and flow cytometry. Results Across independently analyzed cohorts, a CD74-MHC-II-CXCL inflammatory antigen-presenting signature showed concordant elevation in myocarditis. At single-cell resolution in human samples, this signature was concentrated in disease-associated myeloid cells, most notably in an APC/CXCL-high myeloid state defined by strong expression of antigen-presenting and chemokine genes and tightly connected to the IFN/STAT1 response. Within the anti-PD-1 murine model, myocardial injury, inflammatory infiltration, and fibrosis increased, accompanied by greater CD68/CD74 co-immunoreactivity and a higher proportion of macrophage-like cells with detectable surface CD74/CLIP staining. Anti-CD74 antibody administration was associated with improved cardiac function, reduced myocardial inflammation and fibrosis, lower MHC-II-related and CXCL9/10 expression, and lower serum inflammatory cytokine levels. In further experiments, IFN-γ neutralization and JAK inhibition were associated with lower cardiac STAT1-related signaling, CIITA/CD74/MHC-II-related markers, and CXCL chemokine expression, together with reduced myocardial injury. Conclusion Taken together, these findings support the presence of a coordinated myeloid inflammatory program involving IFN-γ/JAK-STAT1, CIITA/CD74/MHC-II, and CXCL chemokines in ICI-associated myocarditis. CD74 is a prominent disease-associated component of this program, and the early preventive anti-CD74 intervention findings provide proof-of-concept support for its further evaluation as a candidate pharmacological target. Its efficacy in established myocarditis remains to be determined.