Orthopoxviruses are large DNA viruses which can cause disease in numerous host species. Today, after eradication of Variola virus and the end of vaccination against smallpox, zoonotic Orthopoxvirus infections are emerging as potential threat to human health. The most common causes of zoonotic Orthopoxvirus infections are Cowpox virus in Europe, Monkeypox virus in Africa and Vaccinia virus in South America. Although all three viruses are genetically and antigenically closely related, the human diseases caused by each virus differ considerably. This observation may reflect different capabilities of these viruses to modulate the hosts' immune response. Therefore, we aimed at characterizing the specific cytokine response induced by Orthopoxvirus infection in vitro. We analysed the gene expression of nine human pro-inflammatory cytokines and chemokines in response to infection of HeLa cells and could identify an upregulation of cytokine gene expression following Cowpox virus and Monkeypox virus infection but not following Vaccinia virus infection. This was verified by a strong induction of especially IL-6, IL-8 and CXCL1 secretion into the cell culture supernatant following Cowpox virus infection. We could further show that supernatants derived from Cowpox virus-infected cells exhibit an increased chemotactic activity towards monocytic and macrophage-like cells. On the one hand, increased cytokine secretion by Cowpox virus-infected cells and subsequent monocyte/macrophage recruitment may contribute to host defence and facilitate clearance of the infection. On the other hand, given the assumed important role of circulating macrophages in viral spread, this may also point towards a mechanism facilitating delivery of the virus to further tissues in vivo.
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