Myxoma and vaccinia viruses exploit different mechanisms to enter and infect human cancer cells.

Myxoma (MYXV) and vaccinia (VACV) viruses have recently emerged as potential oncolytic agents that can infect and kill different human cancer cells. Although both are structurally similar, it is unknown whether the pathway(s) used by these poxviruses to enter and cause oncolysis in cancer cells are mechanistically similar. Here, we compared the entry of MYXV and VACV-WR into various human cancer cells and observed significant differences: 1--low-pH treatment accelerates fusion-mediated entry of VACV but not MYXV, 2--the tyrosine kinase inhibitor genistein inhibits entry of VACV, but not MYXV, 3--knockdown of PAK1 revealed that it is required for a late stage event downstream of MYXV entry into cancer cells, whereas PAK1 is required for VACV entry into the same target cells. These results suggest that VACV and MYXV exploit different mechanisms to enter into human cancer cells, thus providing some rationale for their divergent cancer cell tropisms.