Self-tolerance is the ability of the immune system to recognize self-produced antigens as a non-threat while appropriately mounting a response to foreign substances. This balance of immunological defense and self-tolerance is critical to normal physiological function and overall health. Self-tolerance mechanisms play roles in cancer, auto-immune diseases, colonization by commensal bacteria, and pregnancy.
As lymphocytes develop, the receptor sequences are generated completely at random. This indiscriminate approach results in such a diverse variety of receptors that nearly any antigen the immune system encounters may be recognized. However, this amazingly broad repertoire inevitably contains receptors that can recognize endogenous antigens, self-made molecules which the immune system should not attack. In order to reduce the scope of lymphocytes recognition and subsequent attack of foreign antigens, the immune system employs a series of checks known generally as "self-tolerance". Self-tolerance regulation of immune effector cells can be divided into two mechanisms termed "central tolerance" and "peripheral tolerance", depending on where they take place. Central tolerance occurs in the organ of maturation for the respective lymphocyte, the thymus for T-Cells and bone marrow for B-Cells. Peripheral tolerance occurs outside the organ of maturation, at the site of antigen recognition where the T-Cells and B-Cells would ultimately begin to elicit an immune response. Specifically, this can occur in the circulation, lymph node, lymph organ, or other tissues. Read More »
|Tolerance||Central Mechanism||Peripheral Mechanism|
|T-Cell||Regulatory T-Cell, Clonal Deletion (Apoptosis)||Suppression, Deletion|
|B-Cell||Receptor Editing, Clonal Deletion (Apoptosis)||Anergy|
Stringent T-Cell screening is critical as they can further differentiate into cytotoxic lymphocytes (CTL), directly killing cells they recognize via their T-Cell receptor (TCR). T-Cell central tolerance is controlled in the thymus during T-Cell maturation. After initial positive selection for CD4 or CD8, cluster of differentiation markers that bind MHCI or MHCII respectively, the next criteria is negative selection based on TCR recognition of self-antigen. The cortical epithelial cells of the thymus present self-antigen to CD4+ and CD8+ T-Cells. Maturing T-Cells which bind strongly to self-antigens are clonally deleted through apoptosis. T-cells which bind self-MHC complexed antigens within a "Goldilocks" range, not too strongly but not too weakly, are fated to become regulatory T-cells (Tregs). Tregs are important in controlling peripheral tolerance, discussed below.
Despite rigorous selection criteria in the thymus, not all self-antigen reacting T-Cells can be clonally deleted. Some self-reactive T-Cells escape screening and emigrate into the periphery. Peripheral T-Cell tolerance serves as a secondary checkpoint. There are multiple arms to peripheral tolerance. Thymic Tregs, the T-Cells with "Goldilocks" degree of self-antigen recognition, suppress auto-reactive T-Cells. New studies have shown that there is an additional population of Tregs generated in the periphery, conventional CD4+ T-Cells induced to Treg lineage outside the thymus. While still an area of active research, both populations of Tregs have been shown to inactivate self-reacting T-Cells via surface expressed suppressor CD25, and inhibitory cytokines, IL-10 or TGFβ. Additionally, surface receptors can shift self-reactive T-Cell effector cells towards anergy, a state of non-reactivity. Specifically, programmed death 1 (PD-1) receptor and its ligands PD-L1 and PD-L2, as well as cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) activation result in anergic cells. Finally self-reactive T-Cells can be deleted in the periphery through apoptosis, induced by the death receptor Fas and up-regulation of its cognate ligand, FasL.
Much like T-Cells maturing B-cells in the bone marrow which react to self-antigens via their B-Cell receptor (BCR) are restricted from progressing into the periphery. Rather than immediate removal, self-reactive B-Cells have the opportunity to undergo receptor editing. This receptor rearrangement allows for recognition of alternate, non-self-antigens. If receptor editing is unsuccessful, self-reactive B-Cells are negatively selected and clonally deleted through apoptosis as the ultimate means of B-Cell central tolerance.
Also like T-Cells, there is an opportunity for B-cells which escape self-antigen screening in the bone marrow to be controlled in the periphery. Unlike T-cells which can kill autonomously, B-cells require signaling from T-helper cells. T-helper cells are mature effector T-Cells which have further differentiated into to subtypes, Th1 which help intracellular pathogens like bacteria and protozoa, and Th2 which support immune effectors in defense from extracellular pathogens. During an immune response against exogenous threats Th2 populations predominantly complex with B-Cells via MHCII recognition. Lacking Th2 signaling via MHCII, self-antigen recognizing B-Cells receive only one signal from binding of antigen and undergo anergy. This highlights the critical nature of T-cell tolerance mechanisms, as a helper T-Cell providing positive signaling to a self-reactive B-Cell would be highly detrimental. As such helper T-Cells undergo regulatory checks during differentiation and expansion as well.
Self-tolerance is a complicated system of safeguards that may be corrupted at any stage. Autoimmune disorders result from any number of missteps in the self-tolerance system. Some pathogenic states in which autoimmunity has been implicated include: idiotype cross-reactivity, epitope drift, and aberrant BCR-mediated feedback. Self-tolerance errors result in autoimmune disorders such as celiac disease, type-1 diabetes, inflammatory bowel disease (IBD), multiple sclerosis to name a few. Self-tolerance mechanisms also provide barriers to therapeutic interventions. Tissue engraftment after transplant procedures prove difficult as the immune system correctly recognizes allopatric antigens from the donor tissue as foreign. Manipulation of the immune system towards tolerance of transplanted tissue expressing non-self-antigens would be diminish graft rejection. Conversely, immune response to cancer cells is blunted due to appropriate self-tolerance response as cancer cells express self-antigen. In this case, effective immunotherapy against cancer requires breaking of the self-tolerance mechanisms.
A major challenge researchers face when generating antibodies against high homology antigens is overcoming self-tolerance. Generating sufficient response to antigen requires the self-tolerance mechanisms outlined here to be precisely overcome. While this is a daunting task, GenScript has developed proprietary technology to do just that. Our ImmunoPlus Technology utilizes key immunomodulators to circumvent self-tolerance mechanisms and allow B-Cells to generate high affinity antibodies.
Surrogate antibodies are just one instance where antibodies to self-antigens with high homology are needed. Surrogate antibodies can be a means of testing eventual safety and efficacy of human therapeutic antibodies in mouse models. Targeting the correlate antigen in a mouse with a mouse generated antibody can provide some understanding of a humanized antibody in human system can be gained. Specifically, the degree of toxicity and efficacy can be surmised and extrapolated to a human model. The following case study exemplifies the differences between traditional immunization and ImmunoPlus supplemented regimens. The antigen target maintains a 96% homology between the human and mouse sequences. While this resulted in poor titres from regular immunizations, self-tolerance manipulation allows for an approximate 10-100 fold increase in the immune response.
GenScript has over 12 years of experience developing customized antibodies against difficult targets. Our ImmunoPlus technology is available through our Custom mAb and Custom pAb services. You can even utilize ImmunoPlus in our Semi-Custom mAb Production Service which simplifies custom mAb protocols to 3 easy steps. Some other related services include:
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