This week I want to talk about an interesting enzyme: Arginase-I (Arg1) and its metabolic pathway in an immune microenvironment. Arg1 is a cytosolic protein that is involved in the urea cycle. Specifically, it catalyzes the hydrolysis of L-arginine to L-ornithine and urea. It has been shown that the expression of Arg1 by macrophages has an important role in tumor growth. Macrophages that are recruited into the tumor microenvironment (tumor associated macrophages) express high levels of Arg1 resulting in the depletion of arginine – an essential nutrient required for T cell metabolism. Cytotoxic T cells, therefore, can no longer function and inhibit the tumor cells from proliferating. Arg1 is implicated in several inflammatory diseases as well as in autoimmunity. Arg1 plays a significant role in mediating immune suppression and blocking its metabolism is a novel strategy in preventing tumor growth and other inflammation-related conditions.
One hypothesis is that the immune suppressive cells in other immune microenvironments in the body must be similar to the Myeloid-Derived Suppressor Cells (MDSCs) that induce T cell suppression in the cancer microenvironment. The question is, do such immunosuppressive cells express increased levels of Arg1 and act through the Arg1 metabolic pathway? Since I am interested in the brain and neurodegeneration, this hypothesis can be extended to the brain immune microenvironment. Microglial cells in the brain also upregulate Arg1 and are neuroprotective in nature (in a healthy brain). These cells are the resident macrophages of the central nervous system and function by phagocytosing cell debris and toxic misfolded proteins (that eventually form aggregates and lead to neuronal death as seen in Alzheimer’s disease) out of the brain environment. The question now is – do the microglial cells exhibit immunosuppressive behavior by altering their Arg1 metabolism?
Kan et al., 2015, recently showed that CD11c positive microglial cells are immunosuppressive in the CVN-AD mouse model and that immune suppression is caused due to the deprivation of arginine (increased levels of extracellular Arg1 causing decreased levels of total brain arginine). What isn’t explicitly mentioned in this study is that arginine is also the substrate for nitric oxide synthase (NOS) that makes nitric oxide (NO) in an alternate L-arginine metabolic pathway. L-arginine is a substrate for both Arg1 and NOS. The Arg1 pathway polarizes the macrophages to M2 phenotype and the NOS pathway polarizes the macrophages to the M1 phenotype (Rath et al., 2014). The current model of microglial activation in the CNS is limited to these two polarized states, where, the M1 microglia are neurotoxic and the M2 microglia are neuroprotective. Arg1 is upregulated in microglia in the healthy brain and aids in phagocytosis of misfolded proteins and other cell debris. The classical microglial activation is through the M2 phenotype wherein the induced nitric oxide synthase (iNOS) is upregulated thereby accelerating inflammation in the brain (neuroinflammation is one of the hallmark characteristics of several neurological diseases such as Alzheimer’s Parkinson’s, Multiple Sclerosis, Traumatic Brain Injury, etc).
So, if an immunosuppressive cell exists in the brain, is it possible that the immune suppression is regulated through the M2 activation and that M1 activation is absent? In other words, L-arginine is metabolized through the Arg1 pathway and not through the NOS pathway. Other questions to consider: MDSCs upregulate both Arg1 and iNOS – so how does that fit into the two-state polarization model? How does iNOS modulate MDSC activity? We know that increased NO expression by MDSCs increases T cell suppression in the tumor microenvironment. Currently, both Arg1 and iNOS inhibitors are being developed to block the immune suppressive activity of MDSCs in the cancer microenvironment. However, understanding immunosuppression in the brain is still a long way to go and the idea is not widely accepted within the neurobiology community (my understanding from the currently available literature or published studies). Investigating this mechanism in the brain will be useful in developing potential therapeutic strategies for treating neuroinflammation and neurodegeneration.
- Kan MJ, Lee JE, Wilson JG, et al. Arginine Deprivation and Immune Suppression in a Mouse Model of Alzheimer’s Disease. The Journal of Neuroscience. 2015;35(15):5969-5982. DOI:10.1523/JNEUROSCI.4668-14.2015.
- Rath M, Müller I, et al. Metabolism via arginase or nitric oxide synthase: two competing arginine pathways in macrophages. Front. Immunol., 27 October 2014. DOI: https://doi.org/10.3389/fimmu.2014.00532