Arch Virol (1999) 144: 273–285 Feline peripheral blood mononuclear cells express message for both CXC and CC type chemokine receptors E. M. Kovacs 1 , G. D. Baxter 2 , and W. F. Robinson 1 1 Division of Veterinary Pathobiology, School of Veterinary Science, University of Queensland, St. Lucia, Queensland, Australia 2 Department of Parasitology, University of Queensland, St. Lucia, Queensland, Australia Accepted September 7, 1998 Summary. The feline immunodeficiency virus (FIV) causes a disease in cats similar in clinical presentation and disease progression to that of HIV and AIDS. It is now known that, for HIV infection, as well as primary binding of virus to the CD4 receptor, entry and infection of cells requires coreceptors which are members of the chemokine group of G-protein coupled receptors. Because of the similarity of HIV and FIV, we hypothesised that coreceptors are required for the entry and infection of cells by FIV. Using a feline cDNA library derived from a feline IL-2 sensitive lymphocyte cell line, we identified the presence of message for both CC and CXC chemokine receptors. The feline CXCR4 has been shown to facilitate fusion by FIV [44] and we suggest that the feline CCR5 receptor mediates infection of feline cells by M-tropic strains of FIV. Introduction The initial interaction of the human immunodeficiency virus (HIV) with a target cell is through the binding of the virus envelope with the cell surface receptor, CD4 [22, 36, 41]. However, binding of the virus to CD4 alone is not sufficient for viral fusion and entry into the cell [42]; this can only occur by the binding to CD4 in conjunction with a second cell surface receptor [5]. Candidate coreceptors for viral fusion and entry have been identified and are members of the chemokine group of seven transmembrane G-protein coupled receptor family [2, 16]. Cellular tropism is governed primarily by the ability of a particular virus strain to utilise one or more of these cell lineage-specific coreceptors for productive infection [10]. The M-tropic, non-syncytium-inducing (NSI) strains of HIV that are present during the asymptomatic stage of infection [4, 35] utilise the CC chemokine