Enhancement of mucosal immune responses by chimeric influenza HA/SHIV virus-like particles.
To enhance mucosal immune responses using simian-human immunodeficiency virus-like particles (SHIV VLPs) as a mucosal HIV vaccine, we have produced phenotypically mixed, chimeric influenza HA/SHIV 89.6 VLPs and used them to immunize C57B/6J mice intranasally. Systemic and mucosal antibody responses, as well as cytotoxic T cell (CTL) responses, were compared in groups immunized with SHIV 89.6 VLPs or HA/SHIV 89.6 VLPs. Intranasal immunizations were given using VLPs either with or without the addition of the mucosal adjuvant cholera toxin. Total serum IgG, IgG1 and IgG2a, and IgA in saliva, vaginal lavage, lung wash, and tecal extracts were evaluated by enzyme-linked immunosorbent assay (ELISA). The level of serum IgG production to HIV Env was highest in the group immunized with chimeric HA/SHIV 89.6 VLPs. Similarly, mucosal IgA production was also enhanced in the mucosal HA/SHIV 89.6 VLP-immunized group. Analysis of the IgG1/IgG2a ratio indicated that a Th1-oriented immune response resulted from these VLP immunizations. High levels of serum IgG and mucosal IgA against influenza virus were also detected in mice immunized with HA/SHIV VLPs. HA/SHIV 89.6 VLP-immunized mice also showed significantly higher CTL responses than those observed in SHIV 89.6 VLP-immunized mice. Furthermore, a Major Histocompatibility Complex (MHC)-class-I-restricted T cell activation ELISPOT assay showed elevated interferon- [gamma], interleukin-2, and interleukin-12 production in HA/SHIV 89.6 VLP-immunized mice, indicating that phenotypically mixed HA/SHIV 89.6 VLPs can enhance both humoral and cellular immune responses at multiple mucosal sites. Therefore, chimeric HA-containing VLPs represent a potential approach for mucosal immunization for prevention of HIV infection.
Cathy Yao, MD, PhD, began her presentation by explaining that mucosal tissues are major sites of HIV entry and initial infection, and hypothesizing that mucosal immunization could induce remote mucosal site IgA production. In addition, mucosal administration of live, attenuated simian immunodeficiency virus (SIV) or HIV viruses presents safety concerns. Instead, virus-like particles (VLPs) are an attractive approach for developing effective HIV vaccine candidates since these particles can induce both humoral and cellular immune responses, and they can be administered repeatedly. These particles contain Env anchored to the viral envelope, thus retaining native conformation, and although they neither replicate nor contain the HIV genome, they do closely resemble intact HIV virions (see Figure). Finally, VLPs can induce neutralizing antibody and CTL responses.
Yao's lab has been looking most recently at SH1V VLPs that incorporate a human influenza virus component. Yao noted that other VLPs (Rotavirus, Norwalk virus, Papillomavirus, etc.) are capable of stimulating mucosal immune responses and that intranasal immunization with SIV VLPs and SHIV VLPs can induce both systemic and mucosal immune responses. Also, mucosal immunization with formalin-inactivated influenza virus can induce strong protective responses against virus challenge in CD4 T-cell-deficient mice. The VLPs produced in Yao's lab contain truncated envelope protein rather than full-length Env because the truncated version results in better incorporation into VLPs.
Several series of experiments in mice have led Yao and her colleagues to the following conclusions:
* A successfully produced, phenotypically mixed, influenza HA/SHIV 89.6 VLP can be created by using a haculovirus expression system;
* Intranasal immunization of HA-containing SHIV VLPs elicits augmented humoral and cellular immune responses in both systemic and mucosal compartments;
* The common mucosal immunization system was activated since IgA was produced at multiple mucosal surfaces;
* HA-containing SHIV VLPs enhance Th1-type cytokine production (interferon-[gamma], interleukin-2, and interleukin-12) in both systemic and mucosal sites;
* The adjuvant activity of HA was higher than mucosal adjuvant cholera toxin in inducing neutralizing antibodies; and
* Chimeric HA/SHIV VLPs could also induce neutralizing antibodies against HIV 89.6 in CD4 T-cell-deficient mice.
Some studies have shown that intraperitoneal immunization may prime peritoneal B cell precursors for IgA production. These studies suggest that a combination of mucosal and systemic immunization may optimize the mucosal immune response. To test this hypothesis, Yao's group began further studies 1) to compare both humoral and cellular immune responses induced by intranasal or combined intraperitoneal/intranasal SIV VLP immunizations and 2) to confirm the adjuvantic property of phenotypically mixed HA/SIV VLPs other than HA/SHIV 89.6 VLPs. They found that the combination of intraperitoneal and intranasal immunization with SIV VLPs were able to enhance mucosal IgA production as well as CTL responses (as measured by interferon- [gamma] levels). They also found that HA-containing HA/SIV VLPs showed higher immune responses than SIV VLPs and both humoral and cellular immune responses observed in HA/SIV VLP immunized groups were equal to or greater than in those groups immunized with the mucosal adjuvant cholera toxin.
Yao's group is next looking into why the influenza virus component HA enhances the immune responses in immune-deficient mice, as well as whether other immune cells (such as B cells) are also affected. Studies on VLPs, and the best ways to administer them, may lead to the development of safe and effective HIV vaccine candidates.
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Qishi (Cathy) Yao, MD, PhD
Baylor College of Medicine
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|Publication:||Research Initiative/Treatment Action!|
|Date:||Mar 22, 2003|
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