An important proof-of-concept study has shown that is possible to inhibit HIV infectivity by expressing a specific receptor for HIV in a Lactobacillus species that is commonly found in the vaginal microflora.

Of the 40 million individuals presently infected with HIV-1, an estimated 80% were infected by heterosexual transmission, with a higher rate of transmission to females than to males. With the continuing absence of an effective vaccine, researchers are turning to novel anti-viral therapies that can empower women to actively protect themselves against infection.

The bacterial microflora present in the vagina of healthy, non-menopausal women is dominated by the genus Lactobacillus. Lee and colleagues obtained naturally occurring lactobacilli isolates from vaginal swabs taken from healthy volunteers and, after evaluation for favourable growth and colonization ability, Lactobacillus jensenii was selected as the experimental strain.

In this initial study, Lee et al. decided to express the HIV-specific CD4 receptor in L. jensenii. The coding sequences for the first two extracellular domains of CD4 (2D CD4), which are sufficient for high-affinity binding to the gp120 extracellular glycoprotein of HIV, were modified by PCR to conform to the codon usage in lactobacilli, then cloned into a Lactobacillus expression plasmid and protein expression was optimized. A CD4 capture ELISA and HIV-1 gp120-binding assay were used to confirm that the recombinant 2D CD4 was in the correct conformation and was biologically active.

Could the 2D CD4 expressed by L. jensenii block the infectivity of HIV? In an HIV entry assay using a reporter virus, 2D CD4 purified from L. jensenii and conditioned culture media containing secreted 2D CD4 were both shown to inhibit HIV-1 infection of HeLa cells in a dose-dependent manner. In follow-up assays, Lee et al. looked at the effects of co-incubating intact recombinant L. jensenii with both a laboratory strain of HIV-1 and a natural strain. The recombinant lactobacilli were shown to inhibit the entry of the laboratory strain by 95% and the natural strain by 55%. As natural strains of HIV-1 are known to be less sensitive to CD4 inhibition, this result is modest yet still statistically significant.

This in vitro demonstration of the inhibition of HIV-1 infectivity by a recombinant constituent of the vaginal microflora is an important proof-of-principle study in the search for novel methods to halt the HIV epidemic. As yet, the engineered lactobacilli are a long way from clinical development. However, Lee and colleagues have already gone on to achieve stable integration of the 2D CD4 coding sequence into the L. jensenii chromosome, thereby circumventing the problem that the antibiotic-resistance genes present on the expression plasmid would pose for clinical trials. The fact that in a recent issue of PNAS another team report on BMS-378806, a new small- molecule inhibitor that blocks the docking of HIV-1 to CD4, reflects the fact that the efforts of HIV researchers to thwart HIV-1 are not wholly devoted to the search for a vaccine, and all aspects of the infection and transmission cycle are being investigated for potential new targets.