Lymphocyte repertoire selection and intracellular self/not-self discrimination: historical overview

Immunological self/not-self discrimination is conventionally seen as an extracellular event, involving interactions been receptors on T cells pre-educated to discriminate, and peptides bound to major histocompatibility complex proteins (pMHCs). Mechanisms by which not-self peptides might first be sorted intracellularly to distinguish them from the vast excess of self-peptides have long been called for. Recent demonstrations of endogenous peptide-specific clustering of pMHCs on membrane rafts are indicative of intracellular enrichment before surface display. The clustering could follow the specific aggregation of a foreign protein that exceeded its solubility limit in the crowded intracellular environment. Predominantly entropy-driven, this homoaggregation would co-localize identical peptides, so facilitating their collective presentation. Concentrations of self-proteins are fine-tuned over evolutionary time to avoid this. Disparate observations, such as pyrexia, and female susceptibility to autoimmune disease, can be explained in terms of the need to cosegregate cognate pMHC complexes internally prior to extracellular display.

💡 Research Summary

This paper revisits the fundamental problem of self‑non‑self discrimination in the immune system, proposing that the decisive sorting of foreign peptides occurs not only at the extracellular interface of T‑cell receptors (TCRs) and peptide‑MHC (pMHC) complexes, but also within the crowded intracellular milieu. After a concise historical review of classic concepts—clonal selection, positive and negative thymic education—the authors focus on recent experimental evidence showing that pMHC molecules bearing identical foreign peptides tend to cluster in lipid‑raft domains of the plasma membrane. They argue that such clustering originates from a physicochemical process: when a foreign protein is expressed at high levels or under stress, it can exceed its solubility limit in the cytosol, leading to entropy‑driven homo‑aggregation. This aggregation co‑localizes identical peptide fragments, allowing multiple MHC molecules to load the same peptide simultaneously, thereby forming a dense pMHC cluster that amplifies TCR signaling upon surface presentation.

In contrast, self‑derived proteins have been evolutionarily fine‑tuned—through regulated expression, stability, and intracellular concentration—to remain below their solubility thresholds, thus avoiding aggregation and the formation of immunogenic clusters. The authors extend this model to explain two seemingly unrelated phenomena. First, fever (pyrexia) is interpreted as a physiological means to transiently lower protein solubility, promoting the aggregation of foreign proteins and accelerating the formation of immunogenic pMHC clusters, thereby enhancing immune activation. Second, the higher prevalence of autoimmune diseases in females is linked to X‑chromosome dosage effects and hormonal fluctuations that destabilize the delicate balance of self‑protein concentrations, increasing the risk of inadvertent aggregation and subsequent presentation of self‑peptides.

By integrating concepts from polymer physics, thermodynamics, and evolutionary biology, the paper offers a unifying framework that complements traditional extracellular models. It suggests that intracellular solubility control is a primary gatekeeper of immunogenicity, with implications for vaccine design (e.g., engineering antigens to favor controlled aggregation), immunotherapy (targeting aggregation pathways), and the development of therapeutics aimed at modulating fever or sex‑specific protein homeostasis. The authors conclude with a call for advanced imaging of pMHC clustering in live cells, systematic screening of aggregation modulators, and quantitative studies of solubility differences across age, sex, and disease states to validate and extend this intracellular discrimination paradigm.