A human mutant CD4 molecule resistant to HIV-1 binding restores helper T-lymphocyte functions in murine CD4-deficient mice

CD4 is a cell surface glycoprotein that acts as a co-receptor for the T cell antigen receptor by binding to a non-polymorphic portion of MHC molecules. CD4 also functions as a receptor for human immunodeficiency virus type-I (HIV-1) because the viral envelope glycoprotein gp120 binds to CD4 with a high affinity. We have previously demonstrated that introduction of mutations into CD4 abolished the binding of gp120 and prevented HIV-1 from entering cells and spreading. However, whether introduction of such mutations into CD4 causes decreased binding to MHC and loss of function is yet to be determined. We generated transgenic mouse lines by injecting a mutant human CD4 (muthCD4) gene under a murine CD4 enhancer/promoter to ensure tissue and stage specific expression. To exclude the influence of endogenous murine CD4, transgenic mice were crossed with murine CD4-targeted mice to produce muthCD4 transgenic mice lacking endogenous CD4 (muthCD4TG/KO mice). In these mice, T lymphocytes expressing muthCD4 expanded and matured in the thymus and were present in the spleen and lymph nodes. They also activated B cells to mount an antibody response to a T-dependent antigen. The results from this study suggest that a human variant of CD4 modified to be resistant to HIV-1 binding can rescue the signaling for T cell development in the thymus in vivo, having helper T cell functions. Thus, further characterization of muthCD4 molecules should open the way to new HIV treatment modalities.

Construction and characterization of M13 bacteriophages displaying gp120 binding domains of human CD4.

The envelope glycoprotein, gp120, on the surface of HIV interacts with the human CD4 molecule and thus helps the virus in gaining entry into the T-helper cells. To display the gp120 binding domains of human CD4 on the surface of the bacteriophage M13, two types of vectors have been constructed. In these, the first 176 amino acids of the human CD4 have been fused with the minor coat protein, gIIIp, of M13 bacteriophage for surface display. The Western blot analysis revealed that using the phage based vector, M13CD41923, all the copies of gIIIP (3-5 per virion) were present as fusion protein indicating multivalent display. In the phagemid based vector, phage particles were produced only upon infection of the cells carrying pVCCD43426, with the helper phage, M13KO7. Thus these phage particles carried both, the fusion protein as well as the unfused gIIIp, as shown by Western blot analysis. The presence of large amount of unfused gIIIp ensured that the phage particles did not display more than one fusion protein per phage particle, thus leading to monovalent display. Phage particles produced by both vectors could be captured on immobilized gp120, thereby showing that the displayed CD4 domains were functional.