By Sashini Ranawana
With the majority of hereditary diseases, it has always been a case of discerning the extents to which genetic and environmental factors play a part – the famous ‘Nature versus Nurture’ conundrum. Autoimmune disorders are no exception. T lymphocytes containing mutations in immuno-regulatory genes usually require activation by external triggers to express their autoaggressive phenotype. This sets into motion a faulty genetic pathway, which ultimately results in immune cells recognizing and bringing about a response against healthy ‘self’ cells (Rosenblum, Remedios and Abbas, 2015). One of the widely acknowledged mechanisms of Multiple Sclerosis (MS) onset is a suitable example of this. Individuals with a high genetic susceptibility to MS usually have T effector cells containing a mutant HLA-DRB1 gene; the consequence of this being a reduced tolerance to autoantigens. In many instances, following infection of immune cells by the Epstein-Barr virus, B cells activate these T cells by presenting them with the autoantigen myelin basic protein (MBP). The eventual diagnosis is a neurodegenerative disorder in which the body’s immune system attacks and destroys the myelin sheath surrounding neurons in the Central Nervous System (Guan et al, 2019).
Autoimmunity was once considered to be a sealed fate, but we now have evidence to the contrary. The field of epigenetics (modifications to genes above the level of DNA) has shown that cellular pathways are not permanent, and patterns of gene expression can be manipulated. This is the principle that is now being applied to combat autoimmune diseases. By repeatedly introducing specific soluble proteins to effector T lymphocytes, genetic circuits can essentially be rewired to reduce the risk of intolerance. T cells can become anergic. The feasibility of this theory was tested in a recent study, when Tg4 mice T cells, which usually react against the Ac1-9 peptide of MBP, were exposed to the analogous peptide Ac1-9[4Y]. What followed was an alteration in both the arrangement of chromatin, and the transcriptional activity of genes within the cells (Bevington et al, 2020; University of Birmingham, 2020).
Any genotypic changes brought about by the antigen tolerization process are reflected in the phenotype of lymphocytes, and ultimately in their immune function. Tg4 T cells continually exposed to the Ac1-9[4Y] antigen experience increased signaling to the PD-1 inhibitory receptor, and subsequent activation of the Cbl-b repressive factor. Cbl-b, a ubiquitin ligase enzyme, has an established role in the suppression of protein kinase C (PKC), phosphoinositide 3-kinase (PI3K) and phospholipase C (PLC) pathways, all of which are essential to the cell’s pro-inflammatory status. The ubiquitination of PKCθ (a molecule in the PKC pathway) and PI3K reduces the extent of CD28 and T cell receptor (TCR) signaling to transcription factors such as AP-1 in the nucleus. This block effectively lowers the activation ability of many standard immune response genes, rendering the immune reaction against the peptide ineffective. Further insight into the nature of this signal disruption was provided when tolerized cells were exposed to the A23187 calcium ionophore, which is known to intensify signaling downstream of PLC and PKC molecules. Cells treated in this way had no perceptible indication of gene expression reprograming. Cbl-b evidently functions to block signal transduction close to the initial site of TCR/CD28 activation (Bevington et al, 2020).
Through tolerization, T cells not only experience changes in gene expression: they also undergo long-term adjustments to chromatin states. In this way, genes are made more transcriptionally active or inactive through their altered accessibility to transcription factors. One key finding of the study was that the reduction in TCR/CD28 signaling encouraged the epigenetic modification of histones, which eventually led to the creation of multiple accessible chromatin domains, known as DNase I hypersensitive sites (DHSs). These loose regions of chromatin were found to contain a number of immune-suppressive genes such as Ctla4. mRNA sequencing of these genes indicated that they had become more inclined to activation than immune response genes. It is this imbalance that brings about anergy in T cells. The signal that was obtained from these DHSs remained significantly high for a period of up to three weeks after exposure to the antigen, providing evidence that epigenetic changes are maintained even after cells have become tolerized (Bevington et al, 2020).
Cellular models are only the first step in understanding the potential of T cell anergy in the treatment of autoimmune diseases. Clinical studies are needed to ascertain whether this theoretical approach produces significant results in human biological systems. This is where the focus has now shifted. By considering the overall immune response, researchers have deduced that antigen-presenting dendritic cells play a vital role in tolerization: through the presentation of peptide fragments, also called apitopes, to autoaggressive CD4+ T cells. Any mature T cells consequently formed no longer respond against the protein. This was the mechanism observed with MS patients in a recent trial. When repeatedly treated with ATX-MS-1467, a mixture of four different MBP peptides, the number of T1 gadolinium-enhancing lesions (an indication of inflammation in the brain) considerably decreased (Chataway et al, 2018). This suggests that the cellular pathways leading to tolerance in T cells prevent disease progression even in multi-cellular systems. We are now seeing examples where this form of therapy is being extended to other diseases. In the case of Graves’ hyperthyroidism, patients were continuously injected with ATX-GD-59, made up of two peptides from the thyrotrophin receptor. Tolerance to this protein was achieved, resulting in a significant reduction in free triiodothyronine and autoantibody levels (Pearce et al, 2019). The results of these trials are encouraging and have revealed that antigen-specific immunotherapy does have a future as a therapeutic treatment against autoimmunity.
Despite these promising outcomes, there are still concerns over the consequences of manipulating the immune system. In a study focused on experimental allergic encephalomyelitis, a disorder very similar to MS, inducing tolerance in T cells led to an exacerbation of the condition. It was understood that the exposure to a myelin oligodendrocyte glycoprotein (MOG) peptide in common marmosets increased the activation of Th2 cells, which stimulated the production of autoantibodies against the peptide. In this way, tolerance decreased cell-mediated autoimmunity, but unexpectedly increased humoral autoimmunity (Genain et al, 1996). It is clear that further studies need to focus on whether there is a similar risk in large human cohorts. This is a matter of ultimate importance. The emergence of an immunotherapy such as this, i.e. one which does not weaken the normal immune response, is too valuable an opportunity to pass up.
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