The efficacy of mucosal HIV vaccines has been severely limited by their inability to cross epithelial barriers. A University of Maryland research team led by Dr. Xiaoping Zhu, Li Lu, and including Jackson Laboratory professor Derry Roopenian, Ph.D., has developed a vaccine that breaks through those barriers and protects mice against HIV (Lu et al. 2011). This breakthrough may lead to the development of a similar vaccine in humans.
The majority of HIV-1 infections are transmitted through genital or other mucosa. From there, they rapidly spread to distant mucosal and systemic lymph tissues. The several days that HIV-1 viruses are in the mucosa are an ideal time to target them and prevent them from spreading. However, the physical architecture of mucosal epithelia limits the ability of vaccine antigens to contact immune effector cells (including T, B, and antigen-presenting cells) within the lamina propria (the thin vascular layer of connective tissue beneath mucosal epithelia).
Mucosal immune responses to HIV-1 might be improved if a vaccine could be engineered to deliver viral antigens efficiently across mucosal barriers. To produce such a vaccine, the Zhu/Lu team enlisted the help of the "Fc receptor, IgG, alpha chain transporter" (Fcgrt; common name: neonatal Fc receptor, or FcRn). FcRn is a transporter molecule expressed in a variety of cells and tissues, including the mucosal epithelia of adult animals. It binds to the Fc domain of maternal IgG and transports it across polarized placental or intestinal epithelial cells, which then deliver it and confer pathogen immunity to the fetus before it develops its own immune system. Also, FcRn extends the half-life of IgG antibodies by recycling them through endothelial and other cells.
The researchers hypothesized that a fusion protein containing a portion of the IgG Fc binding site and an HIV antigen would be transported by FcRn through mucosal epithelia to underlying antigen-presenting cells. They had already demonstrated that a fusion protein containing a portion of the IgG Fc binding site and a herpes virus antigen is transported by FcRn and induces protective immunity to a herpes challenge (Ye et al. 2011).
To determine if an HIV-IgG fusion protein would be transported by FcRn and elicit B and T cell immune responses to HIV-1, the research team fused the p24 protein from HIV Gag to an IgG Fc heavy chain. They administered this Gag-Fc fusion protein intranasally to C57BL/6 (B6) mice and found that it is indeed transported across the nasal, lung and tracheal epithelia. In contrast, when administered to control mice (hereafter denoting FcRn knockout B6.129X1-Fcgrttm1Dcr/DcrJ (003982) mice, B6 mice immunized with mutant Gag-Fc (which cannot bind FcRn), and B6 mice immunized with Gag alone), it was poorly transported across these epithelia, indicating that FcRn mediates the transport.
The researchers then immunized B6 mice intranasally with Gag-Fc combined with immunostimulatory DNA rich in CpG motifs (TLR9 agonists) and found that these mice produce significantly more Gag-specific serum and/or mucosal IgG than control mice. These results indicated that a Gag-Fc/CpG combination elicits a strong FcRn-mediated B cell response. Additionally, the spleens of the Gag-Fc/CpG-immunized mice were found to contain 10-15 times more IFNG-producing CD4+ and CD8+ T cells than control mice, indicating that Gag-Fc/CpG immunization also induces a strong FcRn-mediated T cell response to HIV.
Several lines of evidence convinced the Zhu/Lu team that FcRn-mediated mucosal immunization induces viral protection at remote sites: B6 mice booster-immunized intranasally with Gag-Fc and, four weeks later, intravaginally challenged with a virulent vaccinia virus (rVV-Gag) have much lower viral loads and smaller uteri (presumably due to less edema, hemorrhaging, and inflammation) than control mice. B6 mice booster-immunized intranasally with Gag-Fc develop significantly greater B cell responses in lung mediastinal lymph nodes than do control mice. And, B6 mice booster-immunized with Gag-Fc have significantly higher Gag-specific IgG levels in vaginal wash and lung lavage fluids than do control mice. They also have significantly higher T cell responses (higher numbers of IFNG-producing CD4+ and CD8+ T cells) in remote sites (vaginal) than control mice.
Several lines of evidence led the researchers to also conclude that FcRn-mediated mucosal immunization produces long-lasting memory B and T cell immunity: Four months after being booster-immunized with Gag-Fc, B6 mice re-stimulated with the HIV Gag antigen have significantly more long-lasting memory B cells and long lasting bone marrow plasma cells that secrete Gag-specific IgG antibodies than do control mice. When stimulated in vitro with the HIV Gag antigen, the splenocytes of these mice produced significantly more CD4+ and CD8+ T cells than did those of control mice. And, four months after being booster-immunized with Gag-Fc, B6 mice challenged intravaginally with rVV-Gag antigen had significantly smaller viral loads than control mice.
Collectively, the findings by Drs. Zhu, Lu, and their colleagues indicated that, in mice, an intranasally administered vaccine containing an HIV Gag antigen fused to a modified murine Fc IgG segment is effectively transported by FcRn across mucosal epithelia and induces long-term HIV Gag-specific memory B and T cell immunity – both at sites of and distal to infection. If a similar vaccine is shown to be equally effective in humans, it could be a powerful weapon in the fight to stop the HIV/AIDS epidemic.
The efficacy of antibodies depends on their serum half-lives. The serum half-life of IgG is extended by FcRn via the FcRn rescue pathway. Because mouse and rat FcRn differ considerably from human FcRn, their value in developing IgG-based therapies is limited. JAX professor Derry Roopenian, Ph.D., circumvented this limitation by developing the following two mouse strains:
Both are deficient for mouse FcRn. The first expresses human FcRn and can be used to study the half-lives of human/humanized antibodies (especially Fc-engineered antibodies) in vivo, screen the half-lives of Fc-fusion and albumin-fusion proteins, and perform pharmacokinetic studies. The second can be used as a control (as it was in the study just described).