CD4 and CD8 in T cell lineage commitment: alterations induced by expression of a CD8/CD4 chimeric transgene

Nef from a primary isolate of human immunodeficiency virus type 1 lacking the EE155 region shows decreased ability to down-regulate CD4

The human immunodeficiency virus (HIV)nefgene encodes a 27 kDa myristoylated cytosolic protein that has an important role in the pathogenesis of AIDS. One function of Nef is the down-regulation of CD4 and MHC class I surface molecules in HIV-infected cells. Nef directly isolated from an infected individual (KS2), who could be defined as a long-term non-progressor, was compared with Nef from a standard laboratory strain, HIV-1 NL4-3. KS2 Nef protein was characterized by its lowered ability to down-regulate CD4, while still maintaining the ability to down-regulate MHC class I. The ability of KS2 Nef to down-regulate CD4 was more prominent when CD4 was measured 2–3 days after transfer of thenefgene to the target cells, and also when the effect was measured in CD4+-enriched primary T cells. The amino acid sequence analysis indicated that the most notable feature of KS2 Nef was lack of the two glutamic acids: the EE155region. When the EE155region was added to KS2 Nef, the CD4 down-regulation ability was increased almost to the level of NL4-3 Nef. Conversely, when the EE155region was deleted from NL4-3, its CD4 down-regulation ability was dramatically impaired. These data suggested that the EE155region plays an important role(s) in the down-regulation of CD4 by Nef protein and also that primarynefsequences could be very useful in identifying the original biological functions of Nefin vivo.

Subunit H of the V-ATPase binds to the medium chain of adaptor protein complex 2 and connects Nef to the endocytic machinery

Nef is an accessory protein of human and simian immunodeficiency viruses (HIV and SIV) that is required for efficient viral infectivity and pathogenicity. It decreases the expression of CD4 on the surface of infected cells. V1H is the regulatory subunit H of the vacuolar membrane ATPase (V-ATPase). Previously, the interaction between Nef and V1H has been found to facilitate the internalization of CD4, suggesting that V1H could connect Nef to the endocytic machinery. In this study, we demonstrate that V1H binds to the C-terminal flexible loop in Nef from HIV-1 and to the medium chain (mu2) of the adaptor protein complex 2 (AP-2) in vitro and in vivo. The interaction sites of V1H and mu2 were mapped to a central region in V1H from positions 133 to 363, which contains 4 armadillo repeats, and to the N-terminal adaptin-binding domain in mu2 from positions 1 to 145. Fusing Nef to V1H reproduced the appropriate trafficking of Nef. This chimera internalized CD4 even in the absence of the C-terminal flexible loop in Nef. Finally, blocking the expression of V1H decreased the enhancement of virion infectivity by Nef. Thus, V1H can function as an adaptor for interactions between Nef and AP-2.

Commitment to the CD4 Lineage Mediated by Extracellular Signal-Related Kinase Mitogen-Activated Protein Kinase and Lck Signaling

The development of T cells results in a concordance between the specificity of the TCR for MHC class I and class II molecules and the expression of CD8 and CD4 coreceptors. Based on analogy to simple metazoan models of organ development and lineage commitment, we sought to determine whether extracellular signal-related kinase (Erk) mitogen-activated protein (MAP) kinase pathway signaling acts as an inductive signal for the CD4 lineage. Here, we show that, by altering the intracellular signaling involving the Erk/MAP kinase pathway, T cells with specificity for MHC class I can be diverted to express CD4, and, conversely, T cells with specificity for MHC class II can be diverted to express CD8. Furthermore, we find that activation of the src-family tyrosine kinase, p56lck is an upstream mediator of lineage commitment. These results suggest a simple mechanism for lineage commitment in T cell development.