Different Effects of Nef-Mediated HLA Class I Down-Regulation on Human Immunodeficiency Virus Type 1-Specific CD8⁺T-Cell Cytolytic Activity and Cytokine Production

Abstract

A previous study using a Nef-defective human immunodeficiency virus type 1 (HIV-1) mutant suggested that Nef-mediated down-regulation of HLA class I on the infected cell surface affects the cytolytic activity of HIV-1-specific cytotoxic T-lymphocyte (CTL) clones for HIV-1-infected primary CD4 ؉ T cells. We confirmed this effect by using a nef-mutant HIV-1 strain (NL-M20A) that expresses a Nef protein which does not induce down-regulation of HLA class I molecules but is otherwise functional. HIV-1-specific CTL clones were not able to kill primary CD4 ؉ T cells infected with a Nef-positive HIV-1 strain (NL-432) but efficiently lysed CD4 ؉ T cells infected with NL-M20A. Interestingly, CTL clones stimulated with NL-432-infected CD4 ؉ T cells were able to produce cytokines, albeit at a lower level than when stimulated with NL-M20A-infected CD4 ؉ T cells. This indicates that Nef-mediated HLA class I down-regulation affects CTL cytokine production to a lesser extent than cytolytic activity. Replication of NL-432 was partially suppressed in a coculture of HIV-1-infected CD4 ؉ T cells and HIV-1-specific CTL clones, while replication of NL-M20A was completely suppressed. These results suggest that HIV-1-specific CD8 ؉ T cells are able to partially suppress the replication of HIV-1 through production of soluble HIV-1-suppressive factors such as chemokines and gamma interferon. These findings may account for the mechanism whereby HIV-1-specific CD8 ؉ T cells are able to partially but not completely control HIV-1 replication in vivo.

Evasion of cytotoxic T lymphocytes is a functional constraint maintaining HIV-1 Nef expression

“…Given the importance of HIV-1-specific CD8 + CTL in the control of disease (reviewed in [14]) and the role of MHC-I for presenting antigen to CTL, it is hypothesized that Nef mediates viral evasion of HIV-1 from immune control in vivo [15,16]. This hypothesis is supported by in vitro models that demonstrate Nef-mediated interference in clearance of HIV-1 by CTL [17][18][19]. These studies suggest that Nef allows HIV-1 to evade CTL by down-regulating MHC-I and by rendering infected cells relatively resistant to killing.…”

“…Our subsequent study of inhibition of HIV-1 replication evaluated Nef that was physiologically expressed, but also compared virus containing Nef with deleted Nef [19]. Tomiyama et al [18] subsequently reported that a single point mutation in Nef ablating MHC-I down-regulation but not other functions (M20A) could affect the antiviral function of HIV-1-specific CTL, suggesting that MHC-I downregulation is necessary and sufficient. Our current study utilizes this mutation to evaluate the role of MHC-I down-regulation in the interaction of HIV-1 Nef and CTL, exploring the hypothesis that MHC-I down- Table 1.…”

“…A handful of published studies have approached the question of whether Nef protects HIV-1-infected cells from clearance by CTL [17][18][19], including the original demonstration by Collins et al [17] showing protection of infected cells from HIV-1-specific CTL clones. Caveats to this latter study include the expression of Nef under a non-physiologic promoter, and the complete deletion of Nef for comparison.…”

Broadly Increased Sensitivity to Cytotoxic T Lymphocytes Resulting from Nef Epitope Escape Mutations

“…Because Nef has been proposed to mediate resistance of HIV-1 to CTL by down-modulating class I molecules (6,8,9), we investigated whether escape of virus from Nef-specific CTL in vitro altered viral susceptibility to CTL of other specificities. The molecular clone HIV-1 NL4-3.1 was passaged under selective pressure in the presence of two Nef-specific CTL clones (KM3 and STD11, isolated from different infected individuals).…”

“…Given the role of MHC-I in the function of CD8 ϩ CTL and the growing evidence that HIV-1-specific CTL are a determinant of immune control, it has been speculated that Nef potentiates persistence of HIV-1 in the face of the CTL response by down-regulating MHC-I to render infected cells less susceptible to clearance in vivo (6,8,9). In vitro studies have indicated that cells infected with Nef-containing virus are relatively resistant to cytolysis by CTL (6,9), and that replication of Nef-deleted virus is markedly more susceptible to inhibition by CTL than wild-type virus (8). These findings provide indirect evidence for the hypothesis that Nef contributes to the pathogenicity of HIV-1 infection by interfering with immunity against infected cells.…”