On the opposing views of the self-nonself discrimination by the immune system

The Effects of Alternate-Day Corticosteroids in Autoimmune Disease Patients

Introduction

Several studiesdemonstrated that the use of alternate-day corticosteroid therapy maintains control of autoimmune diseases due to the prolongation of their therapeutic effect beyond their metabolic effect, with a significant decrease in side effects in patients. For this reason, the current recommendation for the use of these medications is in a short cycle to avoid adverse effects when used frequently and for prolonged periods of time.

Objectives

To learn variations in serum levels of autoantibodies in autoimmune diseases treated with steroids on alternate days, as well as whether there are differences in the response to them depending on the type of disease. Study Design. A descriptive, retrospective, and cross-sectional study was conducted in which serum autoantibody levels were compared at the time of diagnosis and three months after alternate-day corticosteroid therapy.

Results

We included 106 patients from three autoimmune connective tissue diseases (systemic lupus erythematosus, Sjögren syndrome, and Hashimoto's thyroiditis) and observed a statistically significant decrease in serum autoantibody levels both in patients with lupus and those with Hashimoto's thyroiditis, regardless of the sex of the patients, as well as the type of steroids used.

Conclusions

Treatment with alternate-day corticosteroids achieved a statistically significant decrease in serum autoantibody levels in patients with systemic lupus erythematosus and Hashimoto's thyroiditis.

Minding the gap: The impact of B-cell tolerance on the microbial antibody repertoire

B lymphocytes must respond to vast numbers of foreign antigens, including those of microbial pathogens. To do so, developing B cells use combinatorial joining of V-, D-, and J-gene segments to generate an extraordinarily diverse repertoire of B-cell antigen receptors (BCRs). Unsurprisingly, a large fraction of this initial BCR repertoire reacts to self-antigens, and these "forbidden" B cells are culled by immunological tolerance from mature B-cell populations. While culling of autoreactive BCRs mitigates the risk of autoimmunity, it also opens gaps in the BCR repertoire, which are exploited by pathogens that mimic the forbidden self-epitopes. Consequently, immunological tolerance, necessary for averting autoimmune disease, also acts to limit effective microbial immunity. In this brief review, we recount the evidence for the linkage of tolerance and impaired microbial immunity, consider the implications of this linkage for vaccine development, and discuss modulating tolerance as a potential strategy for strengthening humoral immune responses.

Self-tolerance curtails the B cell repertoire to microbial epitopes

Immunological tolerance removes or inactivates self-reactive B cells, including those that also recognize cross-reactive foreign antigens. Whereas a few microbial pathogens exploit these "holes" in the B cell repertoire by mimicking host antigens to evade immune surveillance, the extent to which tolerance reduces the B cell repertoire to foreign antigens is unknown. Here, we use single-cell cultures to determine the repertoires of human B cell antigen receptors (BCRs) before (transitional B cells) and after (mature B cells) the second B cell tolerance checkpoint in both healthy donors and in patients with systemic lupus erythematosus (SLE) . In healthy donors, the majority (~70%) of transitional B cells that recognize foreign antigens also bind human self-antigens (foreign+self), and peripheral tolerance halves the frequency of foreign+self-reactive mature B cells. In contrast, in SLE patients who are defective in the second tolerance checkpoint, frequencies of foreign+self-reactive B cells remain unchanged during maturation of transitional to mature B cells. Patterns of foreign+self-reactivity among mature B cells from healthy donors differ from those of SLE patients. We propose that immune tolerance significantly reduces the scope of the BCR repertoire to microbial pathogens and that cross-reactivity between foreign and self epitopes may be more common than previously appreciated.

Thoughts engendered by Bretscher's Two-step, Two-signal model for a peripheral self-non-self discrimination and the origin of primer effector T helpers

There are three questions under re-examination here that have been inspired by Bretscher's 'Two-step, Two-signal' model. First, what is the nature of the steps required in order for antigen-responsive cells to become effectors? Second, how does the immune system get started? Third and the most troublesome, what is the mechanism that relates the delivery of the two signals? To answer the first question, Bretscher proposes a pathway that I will place in another context by comparing it with what had been envisaged under the Associative Recognition of Antigen (ARA) model. The second question, how does the immune system gets started, is crucial to our understanding of the self-non-self discrimination. This problem boils down to, what is the origin of the first effector T helper (eTh) cells required to activate all antigen-responsive cells including the T helpers themselves (the primer problem)? To deal with this question, I proposed an antigen-independent pathway to primer eTh. Bretscher presents us with an antigen-dependent pathway to primer eTh. As competing models are precious in clarifying thinking and in guiding experimentation, I felt it important to reanalyse the two models and look for ways to decide between them. The third question deals with the requirement for and the mechanism of associative (linked) recognition of antigen (ARA). The concept of ARA is so compelling at both the experimental and theoretical levels that to save it, a new perspective will be introduced.

Comment on: Ten experiments that would make a difference in understanding immune mechanisms

Melvin Cohn provides a list of experiments to test predictions of his associative antigen recognition (AAR) and Tritope models in relation to standard models of immunity. The manuscript highlights important questions and some decisive experiments testing the competing models. A central aspect of good science is the ability to pinpoint the important questions, something Cohn has, and continues here, to do with great clarity. The problems posed are presented succinctly, although knowledge of Cohn's previous theoretical contributions are needed in some areas to fully understand the path he is taking here. The importance of theory, as championed by Cohn, is a message that needs repeating, as it seems increasingly that immunologists favor description over theory. I briefly comment on each of Cohn's ten experiments and then discuss in detail a critical experiment that is missing from his list-an experiment testing the timing of antigen encounter in ontogeny as a central principle of the self non-self discrimination.

The evolutionary context for a self-nonself discrimination

This essay was written to illustrate how one might think about the immune system. The formulation of valid theories is the basic component of how-to-think because the reduction of large and complex data sets by the use of logic into a succinct model with predictability and explanatory power, is the only way that we have to arrive at "understanding". Whether it is to achieve effective manipulation of the system or for pure pleasure, "understanding" is a universally agreed upon goal. It is in the nature of science that theories are there to be disproven. An experimentally disproven theory is a successful one. As they fail experimental test one by one, we end up with a default theory, that is, one that has yet to fail. Here, using the self-nonself discrimination as an example, how-to-think as I see it, will be illustrated.

The self/nonself issue: A confrontation between proteomes

Defining self and nonself is the most compelling challenge in science today, at the basis of the numerous questions that remain unanswered in the immunology-pathology-therapy debate. The generation of the antibody repertoire, the complicated scenario offered by tolerance and autoimmunity, natural auto-antibodies and their relationship to autoimmune diseases, and positive and negative selection are only a few examples of the unresolved immunological questions. In this context, we proposed that sequence similarity to the host proteome modulates antigen peptide recognition and immunogenicity. Using the available proteome assemblies of viruses, bacteria and higher vertebrates, and applying the low-similarity criterion, we are systematically defining the proteomic similarity of B-cell epitopes already validated experimentally. Here, we report further data documenting that a low similarity to the host proteome is the common property that defines the immunological "nonself" nature of antigenic sequences in cancer, autoimmunity, infectious diseases and allergy.

Why Aire? Compensating for late bloomers

The question under analysis in this commentary is, what was the evolutionary selection pressure that necessitated the ectopic expression of a subset of peripheral self-antigens in the thymus and by peripheral APC? The suggestion is that antigen expression is delayed until after the immune system is responsive.

On the critique by Colin Anderson of 'A reply to Dembic: on an end to the beginning of mis-understanding the immune response'

My proposal of a set of postulates that can be used to guide computer modeling has understandably met with significant criticism at two levels, semantic and conceptual. The major source of contention is my assumption that the sorting of the paratopic repertoire is both necessary and sufficient to explain the evolutionarily selected mechanism for the self-nonself discrimination. While 'necessary' is agreed upon, 'sufficient' is debatable as this commentary illustrates. My essay is in defense of 'sufficiency'.