Cytotoxic CD4 T lymphocytes (CD4-CTL) are important in antiviral immunity. For example, we have previously shown that in mice, CD4-CTL are important to control ectromelia virus (ECTV) infection. How viral infections induce CD4-CTL responses remains incompletely understood. We demonstrate here that not only ECTV but also vaccinia virus and lymphocytic choriomeningitis virus induce CD4-CTL, though the response to ECTV is stronger. Using ECTV, we also demonstrate that in contrast to CD8- CTL, CD4-CTL differentiation requires constant virus replication and ceases once the virus is controlled. We also show that major histocompatibility complex class II molecules on CD11c1 cells are required for CD4-CTL differentiation and for mousepox resistance. Transcriptional analysis indicated that antiviral CD4-CTL and noncytolytic T helper 1 (Th1) CD4 T cells have similar transcriptional profiles, suggesting that CD4-CTL are terminally differentiated classical Th1 cells. Interestingly, CD4-CTL and classical Th1 cells expressed similar mRNA levels of the transcription factors ThPOK and GATA-3, necessary for CD4 T cell linage commitment, and Runx3, required for CD8 T cell development and effector function. However, at the protein level, CD4-CTL had higher levels of the three transcription factors, suggesting that further posttranscriptional regulation is required for CD4-CTL differentiation. Finally, CRISPR/Cas9-mediated deletion of Runx3 in CD4 T cells inhibited CD4-CTL but not classical Th1 cell differentiation in response to ECTV infection. These results further our understanding of the mechanisms of CD4-CTL differentiation during viral infection and the role of posttranscriptionally regulated Runx3 in this process. [ABSTRACT FROM AUTHOR]