Near-germline human monoclonal antibodies neutralize and protect against multiple arthritogenic alphaviruses

Significance Arthritogenic alphaviruses, such as Chikungunya virus (CHIKV) and Mayaro virus (MAYV), cause febrile illness, rash, and a debilitating chronic polyarthritis in humans. Currently, there are no approved vaccines or antiviral therapies for the prevention or treatment of alphavirus infection. Here, we identified and characterized 33 monoclonal antibodies (mAbs) from a CHIKV-convalescent donor that cross-react with other arthritogenic alphaviruses. We demonstrate that five broadly neutralizing mAbs can inhibit multiple arthritogenic alphaviruses and map their epitopes through binding and viral escape mutant analysis. Finally, we show that two mAbs, DC2.M16 and DC2.M357, protect against alphavirus disease in mice. These studies inform our understanding of how the human immune system combats alphavirus infection and may guide the development of antiviral treatments and vaccines. Arthritogenic alphaviruses are globally distributed, mosquito-transmitted viruses that cause rheumatological disease in humans and include Chikungunya virus (CHIKV), Mayaro virus (MAYV), and others. Although serological evidence suggests that some antibody-mediated heterologous immunity may be afforded by alphavirus infection, the extent to which broadly neutralizing antibodies that protect against multiple arthritogenic alphaviruses are elicited during natural infection remains unknown. Here, we describe the isolation and characterization of MAYV-reactive alphavirus monoclonal antibodies (mAbs) from a CHIKV-convalescent donor. We characterized 33 human mAbs that cross-reacted with CHIKV and MAYV and engaged multiple epitopes on the E1 and E2 glycoproteins. We identified five mAbs that target distinct regions of the B domain of E2 and potently neutralize multiple alphaviruses with differential breadth of inhibition. These broadly neutralizing mAbs (bNAbs) contain few somatic mutations and inferred germline–revertants retained neutralizing capacity. Two bNAbs, DC2.M16 and DC2.M357, protected against both CHIKV- and MAYV-induced musculoskeletal disease in mice. These findings enhance our understanding of the cross-reactive and cross-protective antibody response to human alphavirus infections.