ObjectiveArthrogenic alphaviruses such as Ross River virus (RRV) and chikungunya virus (CHIKV) circulate worldwide. This virus class causes debilitating illnesses that are characterized by arthritis, arthralgia, and myalgia. In previous studies, we identified macrophage migration inhibitory factor (MIF) as a critical inflammatory factor in the pathogenesis of alphaviral diseases. The present study was undertaken to characterize the role of CD74, a cell surface receptor of MIF, in both RRV‐ and CHIKV‐induced alphavirus arthritides.MethodsMouse models of RRV and CHIKV infection were used to investigate the immunopathogenesis of arthritic alphavirus infection. The role of CD74 was assessed using histologic analysis, real‐time polymerase chain reaction, flow cytometry, and plaque assay.ResultsIn comparison to wild‐type mice, CD74−/− mice developed only mild clinical features and had low levels of tissue damage. Leukocyte infiltration, characterized predominantly by inflammatory monocytes and natural killer cells, was substantially reduced in the infected tissue of CD74−/− mice, but production of proinflammatory cytokines and chemokines was not decreased. CD74 deficiency was associated with increased monocyte apoptosis, but had no effect on monocyte migratory capacity. Consistent with these findings, alphaviral infection resulted in a dose‐dependent up‐regulation of CD74 expression in human peripheral blood mononuclear cells, and serum MIF levels were significantly elevated in patients with RRV or CHIKV infection.ConclusionCD74 appears to regulate immune responses to alphaviral infection through its effects on cellular recruitment and survival. These findings suggest that both MIF and CD74 play a critical role in mediating alphaviral disease, and blocking these factors with novel therapeutic agents could substantially ameliorate the pathologic manifestations.