Differential pathogenesis and antiviral treatment outcomes of MPXV clade Ib and clade IIb in a BALB/c mouse model

The global emergence of monkeypox virus (MPXV) clade IIb since 2022, together with the recent spread of clade Ib, underscores the continuing public health threat posed by MPXV. Clinical and epidemiological observations suggest that clade Ib infection may differ from clade IIb infection, but the biological basis of these differences remains insufficiently defined. In this study, we established an intranasal BALB/c mouse model of MPXV clade Ib infection and compared its pathogenicity with that of clade IIb. Clade Ib caused lethal disease at an inoculation dose approximately ten-fold lower than that required for clade IIb. Compared with clade IIb, clade Ib infection showed faster disease progression, higher pulmonary viral burden at later stages of infection, and more severe lung pathology. Single-cell transcriptome and cytokine response analyses further uncovered clade-specific differences in pulmonary immune cell landscapes, the distribution of MPXV-positive cells, and the expression patterns of key inflammatory mediators at specific time points. Antiviral evaluation showed that tecovirimat and cidofovir were active against both clades, although a higher dose of cidofovir was required to achieve protection in the clade Ib infection model. Together, these findings establish a susceptible BALB/c mouse model for MPXV clade Ib and provide experimental evidence for clade-dependent differences in MPXV pathogenicity and antiviral treatment outcomes.