FIV infection, in domestic cats, causes a variable immunodeficiency syndrome characterized by recurrent gingivitis-stomatitis, cachexia, wasting, neurology, and an increased incidence of tumor development [1, 4, 48, 76] . In contrast, the ungulate lentiviruses cause diseases reminiscent of chronic inflammatory conditions while infection with the bovine lentivirus seems to be inapparent [71] . The rate of progression of the disease can depend on the genotype of the infecting FIV and is also likely influenced by undefined genetic determinants of the particular host [16] . FIV infection in domestic cats is associated with early robust humoral and cellular anti-viral immune responses, followed by a progressive immune suppression that results eventually in AIDS. The outcome of infection depends on the balance between the viral destruction of the immune system and the ability of the remaining immune system to eliminate the virus. Although the decrease in numbers of CD4+ cells is the hallmark of FIV infection, the virus has been shown to infect a variety of cell types in their respective hosts including CD4 + and CD8 + lymphocytes, B lymphocytes, cells of neuronal lineage and monocyte/macrophage lineage [15, 17, 29] . Joshi et al. (2005) have characterized feline CD4 + CD25 + T regulatory cells that support FIV replication. Recently, Reggeti, Ackerley and Bienzle (2008) have shown that feline dendritic cells express specific viral receptors and are infected productively by FIV [53] . FIV shares a similar pattern of receptor usage to HIV-1; however, CD 134 rather than CD4 is the primary binding partner, and subsequent interaction with the secondary receptor CXCR4 permits cells entry [58, 72, 73] . Differences in pathogenicity have been demonstrated among genetically distinct subtypes of FIV that circulate in domestic cats [14, 16, 49, 63, 68, 73] . On the basis of the analysis of envelope glycoprotein (Env), focusing on the third to fifth variable regions (V3-V5), FIV has been classified into five subtypes [30, 46, 60] a number that should be expected to increase as further studies reveal additional diversity. Recent studies identified distinct groups of FIV isolates from the United States and New Zealand [24, 69] (Figure 2 ).