Immunosupression in HIV/AIDS Mediated by Myeloid Derived Supressor Cells

MDSCs: Gabrilovich & Nagaraj, Nat Rev Immunol 2009

Recently, researchers have found a new subset of immune cells that functions to suppress immunity in multiple diseases. Early research has identified them as myeloid-derived suppressor cells (MDSCs), a heterogenous group of immature myeloid cells, divided into granulocytic or monocytic subsets (CD15 or CD14). Recent research has focused on their role in tumor immunology; Hosoi et al showed that during adoptive CTL (cytotoxic T lymphocyte) treatment of cancer (melanoma), CTLs triggered expansion of MDSCs that eventually outnumbered CTLs, secreted reactive oxygen and nitrogen species, and prevented further proliferation of CTLs. This research in cancer biology shows that immunotherapy and immune response can trigger a counter-regulatory response that dampens the immune system. In a recent paper, Ankita Garg and Stephen Spector tested the roles of MDSCs in viral infection, specifically HIV/AIDS, and found a very similar suppressive effect. As not much is known about the mechanisms behind how MDSCs function in the immune response, these authors used HIV infection of PBMCs and co-culture techniques to explore how MDSCs suppress immunity, covering their expansion, function, and mechanisms of function.

Origin of MDSCs; Gabrilovich & Nagaraj, Nat Rev Immunol 2009

To start, the authors treated PBMCs (peripheral blood mononuclear cells) with heat-inactivated or infectious HIV, or gp120, a HIV envelope glycoprotein responsible for binding to the CD4 co-receptor on a helper T cell and allowing the HIV envelope glycoprotein gp41 to contact the host cell membrane and promote viral fusion (see HIV and Neurocognitive Dysfunction). Both infectious HIV and inactivated HIV/gp120 were able to induce expansion of MDSCs (in this paper gated as CD11b⁺CD33⁺CD14⁺HLA-DR^-/lo; see note), suggesting that viral replication is not necessary of induction of MDSCs. Next, they looked that how gp120 was able to induced MDSC expansion, and found that PBMCs cultured with gp120 conditioned media showed increased an MDSC population, and found increased levels of IL-6, a major inflammatory cytokine, in gp120-treated PBMC supernatants, leading them to hypothesize that IL-6 was responsible for inducing MDSCs. They confirmed this by showing IL-6 neutralization inhibited MDSC expansion. Furthermore, they surmised that IL-6 would be acting through its STAT3 pathway (a major pathway for IL-6 signaling), and found increased levels of phosphorylated STAT3 (pSTAT3) in gp120-treated PBMCs. They showed that IL-6 neutralization completely abolished pSTAT3 expression in gp120-treated PBMCs, showing that IL-6 may cause MDSC expansion via signaling through the STAT3 pathway.


After discovering a probable mechanism behind MDSC expansion during infection, the authors then tested the function of MDSCs and the mechanisms behind their (suppressive) function. They found that when they co-cultured MDSCs expanded under gp120 with CD4 or CD8 T cells, they found that MDSCs were able to reduce the production of IFN-gamma, a key pro-inflammatory and anti-viral cytokine. However, when they co-cultured MDSCs with these T cell subsets in different chambers of a transwell, the suppression of IFN-gamma was abrogated, suggesting that MDSCs may require cell-to-cell contact to suppress the activity of T cells during the immune response to infection. Further, they found that gp120-expanded MDSCs expressed high levels of reactive oxygen and nitrogen species (ROS, iNOS, Arg1); these reactive species have been shown by Nagaraj et al to prevent CD8 T cells from interacting with pMHC (antigen-presenting MHC molecule) and responding to specific antigens by altering the CD8-TCR complex. Inhibiting ROS or iNOS while MDSCs were co-cultured with T cells also abrogated suppression of CD8 and CD4 T cell IFN-gamma production.

Figure 6, D: Expansion of Tregs

Finally, they showed that in addition to suppressing IFN-gamma production, IL-10 production was increased in gp120-expanded MDSC-CD4 T cell co-cultures. IL-10 is a key immunosuppressive cytokine, reducing Th1 cytokine production (such as IFN-gamma) and activation of macrophages, neutrophils, etc, and is also linked to T cell impairment during HIV infection; the ability of MDSCs to upregulate IL-10 further establishes its suppressive role in immunity. Since the authors found that IL-10 production did not come from the MDSCs themselves, they hypothesized that they may be coming from regulatory T cells (Tregs). And not surprisingly, when they co-cultured CD4 T cells with gp120-expanded MDSCs, they found expansion of  CD4⁺CD25⁺FoxP3⁺ Tregs. Again, if the co-culture system was set up using a transwell, where MDSCs were separated from CD4 T cells, Treg expansion was abolished. Thus, the authors concluded that MDSCs may further suppress the immune response by inducing Tregs, which can suppress the immune response in multiple ways.

Through multiple stimulation and co-culture experiments, these authors were able to discover aspects pertaining to MDSC proliferation and function. They found that IL-6 induced MDSC expansion via STAT3 signaling, and that this pathway is active during HIV infection. Furthermore, they elucidated the suppressive role of MDSC by showing that these subset of cells can suppress IFN-gamma production by producing reactive nitrogen and oxygen species to hinder the T cell response and also induce T cell IL-10 secretion. Furthermore, they found that MDSCs, in addition to suppressing immunity directly, also suppress immunity indirectly by stimulating Treg proliferation. Lastly, to show that these in vitro studies were relevant to human HIV infection, they sampled the blood of patients with untreated HIV infection and found an increased population of MDSCs (as well as higher plasma IL-6) compared to HIV-uninfected individuals. Thus, these authors not only have elucidated the role of MDSCs in immune suppression and mechanisms behind their action, they discovered a possible mechanism for the the severe immunodeficiency during HIV/AIDS infection. These new findings give scientists yet another target for HIV treatment, though because relatively little is known about this new subset, it may be a while until scientists can confidently manipulate the immune system and restore normal immune function through regulation of MDSCs. While the findings of this article have provided valuable insight into immunosuppression during HIV infection, further studies will need to investigate MDSC biology in more detail and using in vivo methods, to confirm that these MDSC functions are indeed occurring in human disease and infection.

Note:

CD11b - integrin alpha M, expressed by monocytes (and granulocytes, macrophages)

CD33 - transmembrane receptor specifically expressed by myeloid lineage cells

CD14 - a PRR (pattern recognitino receptor) expressed by monocytes (and macrophages)

HLA-DR - human MHC class II, pathologically low expression on monocytes

For a more in-depth look into MDSCs, visit The MDSC Poster




Primary Source:

Garg, A., & Spector, S. A. (2013). HIV type 1 gp120-induced expansion of myeloid derived suppressor cells is dependent on interleukin 6 and suppresses immunity. The Journal of Infectious Diseases.

http://jid.oxfordjournals.org/content/early/2013/10/07/infdis.jit469.long

Secondary Sources:

Gabrilovich, D.I., and Nagaraj, S. Myeloid-derived suppressor cells as regulators of the immune system. Nat Rev Immunol. 2009 March; 9(3): 162–174.

Abeles R.D., McPhail M.J., Sowter D., Antoniades C.G., Vergis N., Vijay G.K. CD14, CD16 and HLA-DR reliably identifies human monocytes and their subsets in the context of pathologically reduced HLA-DR expression by CD14(hi)/CD16(neg) monocytes: Expansion of CD14(hi)/CD16(pos) and contraction of CD14(lo)/CD16(pos) monocytes in acute liver failure. Cytometry A. 2012 October; 81(10): 823–834.

Hosoi, A., Matsushita, H., Shimizu, K., Fujii, S.-i., Ueha, S., Abe, J., Kurachi, M., Maekawa, R., Matsushima, K. and Kakimi, K. (2013). Adoptive cytotoxic T lymphocyte therapy triggers a counter-regulatory immunosuppressive mechanism via recruitment of myeloid-derived suppressor cells. Int. J. Cancer.

Nagaraj S, Gupta K, Pisarev V, Kinarsky L, Sherman S, Kang L, et al. Altered recognition of antigen is a mechanism of CD8+ T cell tolerance in cancer. Nat Med. (2007) 13:828–35.

Montero, A. J., Diaz-Montero, C. M., Kyriakopoulos, C. E., Bronte, V., & Mandruzzato, S. (2012). Myeloid-derived suppressor cells in cancer patients: A clinical perspective. Journal of Immunotherapy, 35(2), 107-115.

Youn, J.I., Nagaraj, S., Collazo, M., and Gabrilovich, D.I. Subsets of myeloid-derived suppressor cells in tumor-bearing mice. J Immunol. 2008 October 15; 181(8): 5791–5802.