Biology of HIV-Nef in vivo

HIV-Nef is established as an important factor for pathogenicity during HIV infection. The exact role and underlying mechanisms however are still unclear. Several studies have shown that Nef expression in T cells results in increased T cell activation. This Nef-mediated T cell activation has been suggested to result in higher viral replication and increased pathogenicity. Studies performed in cell lines have shown that Nef disturbs TCR signaling, resulting in increased T cell activation. This Nef-mediated T cell activation has been suggested to result in higher viral replication and increased pathogenicity. To examine Nef-mediated T cell activation, we used constitutive Nef transgenic mice, which had been earlier described to be activated. During our studies in these mice we came to the conclusion that the observed activation in this model was caused by homeostatic mechanisms, resulting from Nef-mediated thymic atrophy, rather than by direct Nef-mediated effects on T cells. This suggested that Nef expression does not lead to spontaneous T cell activation upon expression in primary T cells in vivo. This finding was substantiated in conditional Nef transgenic mice (chapter 3) with tightly regulated Nef protein induction in the T cell compartment. We generated these mice in order to circumvent Nef-mediated effects on thymic development and secondary effects on peripheral T cell compartment. When Nef expression was induced, a rapid thymic atrophy occurred, substantiating the Nef-mediated atrophy observed in constitutive Nef transgenic mice. To study effects of Nef on T cell activation after TCR stimulation, T cells of inducible Nef transgenic mice were stimulated by infection of the mice with influenza virus. We showed that Nef expression in T cells leads to a reduced anti-viral T cell response, suggesting a role for Nef in the disturbance of T cell function and possibly TCR signaling. The infection of antigen presenting cells (APCs) by HIV is important for viral spread and replication. Since several studies reported a Nef-mediated stimulation of HIV replication in APCs, we generated mice with inducible Nef expression in antigen presenting cells (APCs) and studied Nef effects in APCs during influenza infections. We showed that Nef expression in APCs results in increased T cell responses. This suggests a role for Nef in the stimulation of T cell activation via APCs, in this way increasing the likelihood of HIV infection, as activated T cells are more prone to HIV infection than naﶥ T cells. With the development of the models we present in this thesis, we made a step forward in Nef biology. The findings that Nef expression in T cells does not lead to spontaneous activation of T cells, that it does disturb T cell function, and that Nef expressing APCs stimulate T cell activation are just the first Nef-mediated in vivo functions to be discovered with these models. The inducible Nef transgenic models provide great tools to verify in vitro described Nef interactions and functions in vivo and will thereby aid to define the crucial mechanisms which define Nef as a pathogenicity factor in HIV infection