Human immunodeficiency virus type 1 entry into murine cell lines and lymphocytes from transgenic mice expressing a glycoprotein 120-binding mutant mouse CD4

Use of murine CXCR-4 as a second receptor by some T-cell-tropic human immunodeficiency viruses

The human CXCR-4 molecule serves as a second receptor for primary, T-cell-tropic, and laboratory-adapted human immunodeficiency virus type 1 (HIV-1) isolates. Here we show that murine CXCR-4 can support the entry of some of these HIV-1 isolates. Differences between mouse and human CXCR-4 in the ability to function as an HIV-1 receptor are determined by sequences in the second extracellular loop of the CXCR-4 protein.

DNA immunization of mice against SIVmac239 Gag and Env using Rev-independent expression plasmids

Simian immunodeficiency virus (SIV) structural gene expression, including gag and env, strictly depends on the interaction of the viral posttranscriptional regulator Rev with its target RNA, the Rev-responsive element (RRE). A small RNA element, termed the constitutive transport element (CTE), located in the 3' portion of simian retrovirus 1 (SRV-1) mRNA, can efficiently substitute for the human immunodeficiency virus (HIV) Rev-RRE interaction, and thus render HIV expression and replication Rev independent. We tested the ability of the SRV-1 CTE to drive the expression of SIVmac239 env and gag from subgenomic constructs designed for possible use in vaccine trials. In vitro expression studies showed that when the SRV-1 sequence is coupled to the SIV gag and env mRNAs, it functions in an orientation-dependent fashion, and leads to strong expression of SIV Gag and Env in human and monkey cell lines; levels of CTE-mediated protein expression were similar to those obtained with a functional Rev-RRE system. On the other hand, in murine fibroblast-like cells, SIV Gag and Env were expressed from constructs at relatively high levels even in the absence of Rev-RRE; nevertheless, their expression was increased by the presence of the SRV-1 CTE. As reported previously for HIV, the murine cell lines appeared to be defective for Rev-RRE activity, and required overexpression of Rev to induce a Rev response. Intramuscular injection of the gag-CTE and env-CTE constructs in BALB/c mice resulted in the expression of the corresponding mRNAs, and the production of anti-Gag and anti-Env antibodies, thus suggesting that these vectors might be used for genetic immunization approaches.

Co-receptors for HIV-1 entry

HIV-1 enters its target cells by fusion at the plasma membrane. The primary cellular receptor for HIV is CD4, but this molecule is insufficient to permit viral fusion. During 1996, the necessary entry co-factors (co-receptors or second receptors) were identified as being members of the seven-transmembrane-spanning receptor family fusin: CXCR4 for T-tropic strains and CCR5, principally, for M-tropic strains. The co-receptor functions of these proteins are inhibited by their natural alpha- and beta-chemokine ligands.

CXCR4/fusin is not a species-specific barrier in murine cells for HIV-1 entry

Since some murine cells expressing human CD4 fail to internalize HIV-1, another block was thought to be located at the level of viral entry in addition to CD4. Recently, CXCR4 was shown to function as a coreceptor for T cell line-tropic HIV-1 entry. Here we demonstrated that cells expressing murine CXCR4 and human CD4 fused with cells expressing the env proteins derived from T cell line-tropic HIV-1 and were infected with T cell line-tropic HIV-1 strains. In contrast, the same cells were not infected with chimeric clones constructed by substitution of monocyte- or macrophage-tropic strain-derived env region or V3 region into T cell line-tropic HIV-1, indicating V3 loop of envelope protein is required for murine CXCR4mediated HIV-1 entry. We conclude that murine CXCR4 is not a species specific barrier to the entry of T cell line-tropic HIV-1.