DCs exposed to HIV-C: Key to better Vaccines?

Recent in vitro experiments in our laboratory revealed significant differences with respect to productive infection of and integration in immature dendritic cells (iDCs) with differentially opsonised HIV (1, 2). The opsonisation pattern of HIV may additionally have profound consequences for the outcomes of the antigen-presenting capacity of dendritic cells (DCs) and their ability to mount an adequate immune response. In this context, we want to study the impact of differential HIV-opsonisation on the antigen-presenting capacity of DCs in vitro in the course of this follow-up project. Additionally, we will compare the antigen-presenting capacity of the commonly used IL-4/GM- CSF-generated DCs with the newly described IL15/GM-CSF-derived DCs (3). IL15-DCs were announced to be more potent in inducing efficient T cell responses, which we also found in preliminary experiments. As demonstrated by investigations of HIV in the last 30 years, antibody responses against the virus are not effective and cellular immune responses not powerful enough to suppress or even control HIV. As shown by Lu et al. (4-6) in vitro and in vivo, DCs exposed to aldrithiol-2 (AT-2)-inactivated HIV or SIV induced a virus-specific CTL response, strong enough to kill HIV-infected CD4 + T cells (4) or control the viral load in SIV-infected monkeys (5) or HIV-infected individuals (6). The decrease of the viral load in the HIV-infected patients was associated with a higher amount of HIV-1-gag-specific CD8 + T cells and HIV-1-specific CD4 + T cells (6). Since in vivo HIV is coated with complement fragments from the initial phases of infection and the immune system responds with a massive, oligoclonal expansion of CD8 + T cells during the acute phase of HIV-1 pathogenesis, we want to expose DCs to already complement-coated (AT-2-inactivated) HIV to induce a more effective cellular immune response compared to non-opsonised HIV. We already found this in preliminary experiments (see below). Due to differences in DC infection and integration with differentially opsonised HIV, we will also investigate the impact of IgG-opsonised HIV on the antigen-presenting capacity of DCs. The idea is a creative approach manipulating DCs in a way to elicit virus-specific cellular responses due to coating of the virus with complement fragments prior loading of the most potent antigen-presenting cells and this has to be taken in consideration, if thinking about a DC-based tool to control HIV.

During the project 'DCs exposed to complement-opsonised HIV: A key to better vaccines?' we characterised the antigen-presenting capacity of dendritic cells loaded with differentially coated (= opsonised) HIV. This is of importance since upon virus entry, non-specific immune responses are triggered in the host and within a short time the innate immune system is stimulated. During acute infection multiple humoral and cellular factors are activated, and co-operate in a complex way to generate an efficient immune response fighting the infection. Among the crucial players of the acute phase immune response is the complement system, which coats the surface of HIV-1 particles immediately and spontaneously after virus entry into the host. The virus is very well protected against complement-mediated lysis, thus opsonised HIV-1 particles accumulate during the acute phase of infection. After appearance of antibodies during the chronic phase, complement deposition is strongly enhanced due to antibody-complement activation. Antigen opsonisation can modulate the capture of antigen, itspresentation and the priming of specific CD8+ T cell (= killer T cell) responses. During the project period we found that complement-opsonisation of HIV-1 and other retroviruses, e.g. Friend murine leukemia virus, acted as a natural adjuvant for dendritic cell-mediated induction of specific killer T cells. This study illustrated that dendritic cells exposed to complement-coated HIV-1 induced more pronounced and functional virus-specific killer T cells, which were able to elicit an antiviral activity against infected CD4+ T cells, the main target cells of HIV-1. The increased induction of killer T cells was not observed when dendritic cells were loaded with non-opsonised HIV-1. Therefore, our findings from this study (published in PLoS Pathogens) are highly relevant for the real infectious situation in the body, since most laboratories do not take into account the opsonisation pattern of the virus and perform experiments with non-opsonised HIV-1, which is unlikely to exist in the body due to the immediate immune responses against HIV-1. In another, yet unpublished study, during the project period, we found that dendritic cells, which were generated in the cell culture using a specific stimulation cocktail (IL-15, GM-CSF) exerted an even higher HIV-specific killer T cell-stimulatory capacity than dendritic cells cultivated under well-established conditions using IL-4 and GM-CSF. Such IL-15 dendritic cells might be better for future vaccination strategies than IL-4 DCs. To summarize this part, our data emphasize an enhancing role of complement for the killer T cell-stimulatory capacity of dendritic cells against retroviruses like HIV and understanding the exact interplay between complement-coated particles and different dendritic cell subtypes is of prime importance for novel vaccination strategies against retroviral infections. Another study showed that in some HIV-positive individuals, high levels of antibodies and low levels of complement fragments coat the HIV surface. Thus, we analyzed the impact of antibody-opsonisation on the antigen-presenting capacity of dendritic cells and found that DCs exposed to antibody-opsonised HIV significantly decreased the HIV-specific CD8+ T cell response compared to the above described efficient CD8+ T cell activation induced by DC loaded with complement-opsonised HIV. This was verified using HIV bearing high surface antibody levels, following incubation in plasma from HIV-infected individuals. These data imply that a strong antibody response after vaccination might weaken the killer T cell response and might result in an infavourable outcome (published in Journal of Allergy and Clinical Immunology).