Wednesday, December 14, 2011

Immunotherapy: Giving Our Bodies a Boost to Fight Infection

The human body’s immune system works tirelessly to keep us healthy. So wouldn’t it be nice if we could lend it a hand in its effort to rid our bodies of all types of pathogens and cancers? Immunotherapy is a type of treatment that seeks to augment our own body’s immune system as it battles certain illnesses and physical conditions. Applications of immunotherapy have been found to be significant in the treatment of cancer, AIDS, autoimmune diseases, and in protecting the body following organ transplantations.

Immunotherapy can be broken down into two main categories: active immunotherapy and passive immunotherapy. Active immunotherapy is designed to stimulate our own immune system so that it can be more effective in combating a disease, such as a cancer vaccine. On the other hand, a passive immunotherapy uses artificially produced components of the immune system, such as antibodies, as a form of treatment. Playing an important role in immunotherapy are certain molecules known as immunomodulators. These molecules, including IL-2, IL-7, IL-12 and interferon, interact with the immune system to induce, enhance, or even suppress the actions of the immune system. More specifically, IL-2 can enhance T, B, and NK cell activation, while IL-7 can promote the development of lymphoid progenitor cells from hematopoietic stem cells. IL-12 is important for the differentiation of ThO cells from naïve T cells and interferon improves antiviral responses. In order to develop new kinds of immunotherapy, researchers must first acquire a sound understanding of how the immune system functions as well as the nature of the particular illness being confronted.

One virus that researchers are currently working to develop an immunotherapy for is human cytomegalovirus (HCMV). HCMV is a common virus that can cause severe complications during allogeneic bone marrow transplants, ones that are between two people who are not twins. HCMV has been linked to many serious symptoms and conditions following transplant surgeries, including organ dysfunction, immunosuppression, acute transplant rejection, chronic transplant rejection, and even death (1). The current goal of HCMV management is to treat or all together prevent disease by using ganciclovir and a live attenuated vaccine called Towne (1). The Towne vaccine, however, has not been found to provide consistent protection against HCMV (2). Thus, medical researchers acknowledge that the discovery of a more effective vaccine against HCMV would be an important step in reducing the number medical complications suffered by transplant recipients.

One idea for a vaccine advanced by J. S. Park and his colleagues at the Catholic University of Korea College of Medicine involves increasing the efficiency of antigen presentation on dendritic cells (DCs) so that an immune response would be triggered against the vaccine. By increasing DC antigen presentation, the researchers hoped to activate CD4+ Th cells and CD8+ cytotoxic lymphocytes (CTLs) against HCMV. Because HCMV is an intracellular pathogen a T cell based (cell-mediated) response is preferable compared to an antibody based (humoral) response, however cell-mediated responses are also more difficult to generate. If successful, this would enable patients to undergo transplant operations with a reduced risk of complications caused by HCMV infection.

The first step that Park and his co-workers took to elicit such a response was to locate a HCMV antigen that would strongly activate DCs. They selected the HCMV structural tegument protein pp65 for this experiment because pp65 is an immunodominant epitope for Th Cell and CTL responses. In order to increase antigen presentation the researchers combined this peptide with the HIV Tat gene which contains a peptide sequence that enables pp65 to cross the plasma membrane into a cell. This recombinant protein was called Tat/pp65N&C.

The second step that the researchers took was to test to see if Tat/pp65N&C could specifically penetrate DCs. In doing so they used green fluorescent protein (GFP) attached to Tat and incubated DCs with this Tat/GFP mixture. The DCs tested showed GFP expression during fluorescence-activated cell sorting (FACS), confirming that Tat did in fact enter this cell type.
The next step was to determine if pp65N&C was actually more effective in generating pp65-specific CTLs compared to pp65N&C without Tat. To do this they stimulated peripheral blood mononuclear cells that contain T lymphocytes with DCs that were pulsed with pp65 with or without Tat. An ELISPOT assay was used to measure the frequency of interferon gamma (IFN-y) producing cells after stimulation since both Th1 and CTLs produce this interferon after stimulation. The researchers discovered that the frequency of CTLs increased 1.6- fold compared to 0.5-fold in Th cells when Tat/pp65N&C was used. These results were still higher in both types when compared to pp65N&C without Tat. This result may be favorable since CTLs serve a crucial role in antiviral responses by mediating the targeted destruction of virally infected cells. At the end of the day, it’s the final results that really count. So the final task in this experiment was for the researchers to generate pp-65 specific CTLs stimulated by DCs that used the Tat/pp65N&C as an antigen and compare them to pp-65 specific CCTLs stimulated by DCs that only used pp65N&C as an antigen. They tested the effectiveness of both types of CTLs for eliciting antibody dependent cell- mediated cytotoxicity (ADCC), and it was found that the T cells induced by DCs pulsed with Tat/pp65N&C showed significantly higher cytoxicity than T cells induced by DCs pulsed with only pp65. These results show that the use of the HIV tat peptide to enhance DC antigen presentation can directly lead to a more successful cell mediated immune response against HCMV in vitro.

The results of this experiment were significant because they laid the groundwork for a possible DC-based vaccine against HCMV. There are, however, many factors yet to be taken into account before the effectiveness of such a vaccine can be determined. One such factor would be the patient’s MHC allotype, which could affect how well the antigen is displayed and whether a vaccine using pp65 could be effective. Nevertheless, the promising results of this study coupled with the fact that this protein can be produced on a large scale at a relatively low cost warrant the continued effort toward developing this vaccine. Future experiments would likely involve the use of animal models to examine the in vivo activity of Tat/pp65N&C since the Park experiment only looked at the in vitro effects of the antigen. Hopefully, continued research will soon lead to a major breakthrough in the development of a vaccine to protect against HCMV-causing complications suffered by some transplant recipients.

Reference: Park, J.S.,Park, S.Y., Cho H., Sohn, H.J., Kim, T.G. (2011). Enhanced Induction of T Cell Immunity Using Dendritic Cells Pulsed with HIV Tat and HCMV-pp65 Fusion Protein in Vitro. Immune Network. 11(3): 182-189.
Other citations:
(1) Bij, W., Speich, R. (2001). Management of Cytomegalovirus Infection and Disease after Solid-Organ Transplantation. Clinical Infectious Diseases. 33: S32-S37.
(2) Jacobson, M.A., Sinclair, E., Bredt, B., Agrillo, L., Black, D., Epling, C.L., Carvidi, A., Ho, T., Bains, R., Girling, V., Adler, S.P. (2006). Safety and immunogenicity of Towne cytomegalovirus vaccine with or without adjuvant recombinant interleukin-12. Vaccine. 25:5311-9.

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