Grouppe Kurosawa: HIV - The Excessive Presence of TGFb During the Primary Immune Response Against HIV (print)

The Excessive Presence of TGFb
During the Primary Immune Response Against HIV

TGFb plays an important role in facilitating healing during periods of localized inflammation. It generates a chemotatic gradient for T cells and monocytes, promotes their activation, and subsequently downregulates their activity in a feedback loop. TGFb's contradictory effects on immune functioning can be traced to its ability to activate resting, immature immune cells, such as T cells, while simultaneously inhibiting memory or activated cells of the same lineage. If TGFb is released inappropriately or accumulates excessively, it can have a pronounced immunosuppressive effect on the immune response to subsequent infections. Immune cells must cross the endothelial cell barrier in order to reach the sites of infection. They do so by binding the endothelial cell receptor E-selectin. E-selectin helps recruit Th1 helper CD4 cells into sites of inflammation. It has no effects on the migration of Th2 cells. Th1 cells secrete IL-2, gamma Interferon, and IL-12 and generally activate the immune response against viruses, parasities, and tumor cells. There has long been an argument amongst scientists that HIV infected persons either lack Th1 cells or the immune hormones secreted by Th2 cells (IL-4, IL-5, IL-6, IL-10) are preferentially secreted during the immune response against the HIV virus. This argument is probably correct, but it takes on another dimension by the in vivo observation that only Th1 cells selectively enter sites of inflammation, such as those harboring viral infections. TGFb specifically inhibits the induction of E-selectin on endothelial cells. During AIDS, the levels of circulating E-selectin molecules positively correlates with disease progression. Pro-inflammatory hormones like TNFa and IL-1 stimulate the adhesion of leukocytes to endothelial cells in the course of their promotion of the inflammatory process. Mice genetically deficient in TGFb suffer from a progressive wasting syndrome characterized by a multifocal infiltration of lymphocytes and macrophages into targeted tissues. The increased adhesion of leukocytes to endothelial veins is the initial lesion. If viral proteins induce an excessive production of TGFb, Th1 cells cannot infiltrate tissues that harbor active viral infections. In AIDS, the natural reciprocity between pro and anti-inflammatory hormones is powerfully disrupted by the viral proteins, thereby providing the virus with a survival advantage.

A reliable independent predictor of HIV disease progression is a defective delayed-type hypersensitivity skin test to different recall antigens. TGFb inhibits both immediate and delayed type hypersensitivity reactions, and is secreted from HIV infected CD4 T cells. The presence of TGFb in cell supernatants positively correlates with the inability of these cells to be stimulated by PPD or tetanus toxoid. TGFb also inhibits the development of B cells, cytotoxic T cells, natural killer cells, and lymphokine-activated killer cells by a multitude of routes. First, TGFb downregulates the c-myc gene, thereby directly inhibiting cellular proliferation and making cells sensitive to apoptosis, or programmed cell death. Second, TGFb deactivates macrophages, which inhibits their ability to scavenge pathogens, process antigens, and release pro-inflammatory, immune activating hormones. Third, TGFb inhibits the expression of pore-forming proteins in cytotoxic CD8 cells, which impairs their ability to destroy virally infected cells. Fourth, TGFb downregulates the expression of both MHC class I and II antigens, which inhibits both humoral and cell-mediated immunity. Fifth, TGFb stimulates the development of CD8 suppressor cells, which inhibit antibody secretion. Although hypergammaglobulinemia is an early symptom of AIDS, the secreted antibodies are biased towards the IgG1 and IgA isotypes. TGFb selectively stimulates the secretion of IgA, while IL-6, an immune hormone released in the presence of NEF, TAT and gp120 stimulates primarily IgG1 secretion. B cells from HIV+ donors proliferate poorly upon stimulation with Staphylococcus aureau Cowan I (SA), a polyclonal B cell activator. This inhibition was highly correlated with the secretion of TGFb from infected CD4 T cells. Clearly, the presence of low affinity antibody to viral proteins is not an indication that a strong primary immune response has been successfully mounted against the HIV virus. Perhaps a SA B cell stimulatory test should join tuberculin skin tests as an early indicator and predictor of immune status in HIV+ individuals.

In addition to inhibiting the activation of various cell types and their migration out of the blood, TGFb, like glucosteroids, inhibits the synthesis of the chemotactic, and HIV receptor antagonizing hormones MIP-la and MIP-1b. TGFb also downregulates the receptor for MIP-la on bone marrow cells. There has been no research on the affect of TGFb on MIP-la/b secretion from CD8 cells. TGFb is also the factor responsible for the inhibition of bone marrow CD34 stem cell development by intact HIV virus or soluble gp120 proteins.

TGFb and Cyclosporin A (CysA) share many of the same immunosuppressive properties. Both TGFb and Cyclosporin A inhibit the inducible activity of the IL-2 gene in T cells through a noncanonical octamer-binding site. Cyclosporin A's ability to stimulate the transcription of the TGFb gene may play an unappreciated role in generating many of the immunosuppressive properties attributed to this drug. TGFb may also be responsible for some of the harmful side effects of Cyclosporin A, such as hypertension and kidney fibrosis. Cyclosporin A, a potent inhibitor of the phosphatase calcineurin, inhibits the development of CD4+CD8+ thymocytes into mature single positive T cells. CD4+CD8 thymocyte development appears to be particularly sensitive to CysA. TGFb, on the other hand, impairs the development of CD4-CD&- (double negative) to CD4+CD8+ (double positive) T cells. The further differentiation of double positive thymocytes to single positive CD8 and CD4 populations is not equally impaired by TGFb. CD8 cells develop normally from the double positive cells that survive TGFb treatment, while TGFb further inhibits the development of CD4 T cells. In TGFb knockout mice, CD4 T cell counts increase dramatically. The TGFb modulating this differentiation pathway is released from thymic stromal epithelial cells. These cells are productively infected with the HIV virus very early after seroconversion. HIV induces apoptotic cell death in CD4+CD8+ and CD4+ medullary thymocytes by an indirect or bystander effect, since the dying cells were not infected with virus. In SCID-hu mice infected with HIV, the depletion of the immature CD4+CD8+ cell population was highly correlated with the thymic viral load. In both infected children and adults, naive CD8 T cells are lost from the blood at the same rate as CD4 cells. Memory or activated CD8 T cells comprise over 80% of T cells in PBMC from individuals with CD4 counts less than 200/ul. Normally, memory CD8 cells constitute 5% of the PBMC in uninfected individuals. The inability to produce adequate numbers of naive CD8 cells inevitably results in a collapse of the cell-mediated immune response against virally infected cells. A loss in both naive and memory CD4 cell populations has also been reported in the latter stages of infection. Since CD4 cell counts decrease at a steady rate of 50/ul per year before disease symptoms appear, a TGFb and viral protein block in CD4+CD8+ and CD4+ development is a reasonable interpretation for the early immune associated with HIV infections.

Macrophages are a primary site of HIV infection and may pick up the virus in areas of inflammation, such as the anal and vaginal mucosa. TGFb is secreted from activated macrophages and is thought to play a role in the central nervous system dysfunctions associated with AIDS. TGFb upregulates HIV replication in macrophages by a mechanism that is not completely understood. When normal macrophages are exposed to T-cell tropic HIV viral strains, they do not establish a productive viral infection. However, if the same macrophages are treated with physiological concentrations of TGFb after infection, the T-cell tropic strains proliferate to the same levels as monocyte tropic strains. Progression of disease is associated with a shift from monocyte tropic to T-cell tropic virus populations. Monocyte tropic strains predominate during early infections and cause the greatest decline in CD4 T cells counts in SCID-hu mice. Since these strains do not replicate in T cells, the decreased CD4 cell count is not due to a direct cytolytic event. Direct T cell-monocyte interactions are necessary for high level HIV production. In the presence of TGFb, tissue macrophages at the sites of infection may pass T-cell tropic strains to T cells thereby accelerating the CD4 T cell decline. Cell-to-cell transmission of virus occurs within minutes and does not necessarily involve the participation of virus particles. HIV+ individuals harbor multiple viral genotypes with distinct cytopathogenic potentials. TGFb stimulated tissue macrophages may provide a stable reservoir for the dissemination of these viruses throughout the body.

TNFa activates many cells that are subsequently deactivated by TGFb. In AIDS, a shift from a Th1 to Th2 CD4 T cells responses against the HIV virus is thought to occur. This immunological shift, so to speak, would be expected to change the immune emphasis or predominance from a cell-mediated to a strictly humoral response. Humoral or antibody-based immune responses alone cannot clear many viral infections. TGFb may accelerate the shift towards the Th2-type response by direct and IL-10 mediated pathways. When cardiac allografts were transfected with the TGFb gene, the secreted protein inhibited the priming of Th0 cells and their conversion to primed Th1 cells. TGFb stimulates macrophages to produce IL-10, and IL-10 deactivates macrophages by blocking the endogenous production of TNFa. IL-10 also inhibits dendritic-T cell cluster formation with either CD4 or CD8 cell populations. TGFb and IL- 10 work synergistically to modulate or turn down the immune response against various pathogens. IL-10 promotes the degradation of cytokine mRNA while TGFb primarily suppresses translation. Both IL-10 and TGFb inhibit NF-kB activation, possibly by stimulating the transcription of the NF-kB inhibitor IkBa and inducing long term antigen-specific anergy in both CD4 and CD8 T cells. Glucosteroids also induce the synthesis of IkBa thereby linking the IL-10 TGFb and hydrocortisone-mediated immunosuppressions to similar pathways. B lymphocytes and PBMC cells from HIV+ individuals release large amounts of IL-10. The production of IL-10 is negatively correlated with the CD4 cell count, and positively correlated with the degree of Th1 dysfunction. Anti-IL-10 antibodies restore Th1 functionality. The synthesis and release of IL-12 from Th1 T cells is also deficient in HIV+ individuals; the defective synthesis of IL-12 can be restored by the administration of gamma interferon, a macrophage activator that is itself inhibited by IL- 10, TGFb and hydrocortisone. TGFb also antagonizes the activity of IL- 12 at the cellular level. IL-12 has been used in clinical trials to restore the defective cell-mediated immune response of HIV+ individuals. If viral TAT, VPR or gp120 proteins chronically stimulate the release of TGFb, the immune response to both the virus and other opportunistic pathogens will be weak, if not completely non-existent.