Ohno's hypothesis and Muller's paradox: sex chromosome dosage compensation may serve collective gene functions

Aggregation-prone peptides from within a non-self-protein homoaggregate are preferred for MHC association: Historical overview

New technologies assist re-evaluation of hypotheses on generation of immune cell repertoires and distinctions of self from non-self. Findings include positive correlations between peptide propensities to aggregate and their binding to major histocompatibility complex (MHC) proteins. This recalls the hypothesis that foreign proteins may homoaggregate in host cytosols prior to releasing their peptides (p) to form pMHC complexes. Clues to this included aggregation-related phenomena associated with infections (rouleaux formation, pyrexia, certain brain diseases). By virtue of 'promiscuous' gene expression by thymic presenting cells - perhaps adapted from earlier evolving gonadal mechanisms - developing T cells monitor surface pMHC clusterings. This evaluates intracellular concentrations of the corresponding proteins, and hence, following Burnet's two signal principle, degrees of self-reactivity. After positive selection in the thymic cortex for reactivity with 'near-self', high-level pMHC clustering suffices in the medulla for negatively selection. Following Burnet's principle, in the periphery low-level clustering suffices for T cell stimulation and high-level clustering again provokes negative selection (immunological tolerance). For evolving intracellular pathogens, fine-tuned polymorphisms of their host species have limited to 'near-self' some mimicking adaptations. It is proposed that while entire pathogen proteins may have evolved to minimize their aggregability, the greater aggregability of their peptides remains partially hidden within. Two-step proofreading mechanisms in prospective hosts select proteins containing aggregable peptide for the generation of pMHC clusters at the surface of presenting cells. Through mutations, some proteins of pathogens and cancer cells tend to converge towards the host 'near-self' that its T cells have auditioned to address.

When few survive to tell the tale: thymus and gonad as auditioning organs: historical overview

Unlike other organs, the thymus and gonads generate nonuniform cell populations, many members of which perish, and a few survive. While it is recognized that thymic cells are "audited" to optimize an organism's immune repertoire, whether gametogenesis could be orchestrated similarly to favor high-quality gametes is uncertain. Ideally, such quality would be affirmed at early stages before the commitment of extensive parental resources. A case is here made that, along the lines of a previously proposed lymphocyte quality control mechanism, gamete quality can be registered indirectly through detection of incompatibilities between proteins encoded by the grandparental DNA sequences within the parent from which haploid gametes are meiotically derived. This "stress test" is achieved in the same way that thymic screening for potential immunological incompatibilities is achieved-by "promiscuous" expression, under the influence of the AIRE protein, of the products of genes that are not normally specific for that organ. Consistent with this, the Aire gene is expressed in both thymus and gonads, and AIRE deficiency impedes function in both organs. While not excluding the subsequent emergence of hybrid incompatibilities due to the intermixing of genomic sequences from parents (rather than grandparents), many observations, such as the number of proteins that are aberrantly expressed during gametogenesis, can be explained on this basis. Indeed, promiscuous expression could have first evolved in gamete-forming cells where incompatible proteins would be manifest as aberrant protein aggregates that cause apoptosis. This mechanism would later have been co-opted by thymic epithelial cells which display peptides from aggregates to remove potentially autoreactive T cells.