Almroth Wright, opsonins, innate immunity and the lectin pathway of complement activation: a historical perspective

Metabolic optimization of adoptive T cell transfer cancer immunotherapy: A historical overview

After prolonged extracorporeal multiplication in physiological culture media, there can be curative infusions of a cancer patient's own cytotoxic T cells (adoptive T cell transfer; ACT), which must achieve efficient activation in potentially adverse tumour microenvironments. With spectacular, yet irregular, success, improvements are needed. Developing lymphoid cells are biologically selected, not only for 'near-self' reactivity (positive selection), but also to avoid self-reactivity (negative selection). Thus, success requires harnessing near-self cells while avoiding extreme autoimmune phenomena. Abrupt metabolic changes accompanying T cell activation to leave the G₀ stage and enter the G₁ stage of the cell cycle (eg enhanced glycolysis) are accompanied by increased transcription of the G0S9 gene that mediates salvage synthesis of NAD⁺ from nicotinamide; the latter has recently been shown to increase the efficiency of ACT. Despite theoretical and experimental advances, there has not been parallel progress in simulating in vivo conditions with culture media that were initially formulated for their positive benefits for tumour cell lines (cell survival and proliferation). Yet for lymphoid cells, inhibition or death (ie immunological tolerance) is as important as their activation and proliferation (immunological response). Thus, use of media optimized for the latter may mask the former. The resilience of established culture protocols may have been partly politically driven. However, unphysiological conditions have sometimes yielded fortuitous insights. Optimization of culture media for specific tissues must consider the nature of problems addressed in research settings and the need to avoid mishaps in clinical settings.

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.

On certain two-signal perspectives of lymphocyte activation and inactivation, thymic G-quadruplexes, and the role of aggregation in self/not-self discrimination

Distinctive "two signal" paths in immunology, taken by researchers with different academic backgrounds, seem to have both contained facets of the truth. Having been influenced by education at a medical school where Almroth Wright's early contributions were not forgotten, the author's "path less followed" led to views that began to gain recognition late in the twentieth century when the intimate relationship between innate and acquired immunity became more apparent.

Immunology's Coming of Age

This treatise describes the development of immunology as a scientific discipline with a focus on its foundation. Toward the end of the nineteenth century, the study of immunology was founded with the discoveries of phagocytosis by Elias Metchnikoff, as well as by Emil Behring's and Paul Ehrlich's discovery of neutralizing antibodies. These seminal studies were followed by the discoveries of bacteriolysis by complement and of opsonization by antibodies, which provided first evidence for cooperation between acquired and innate immunity. In the years that followed, light was shed on the pathogenic corollary of the immune response, describing different types of hypersensitivity. Subsequently, immunochemistry dominated the field, leading to the revelation of the chemical structure of antibodies in the 1960s. Immunobiology was preceded by transplantation biology, which laid the ground for the genetic basis of acquired immunity. With the identification of antibody producers as B lymphocytes and the discovery of T lymphocytes as regulators of acquired immunity, lymphocytes moved into the center of immunologic research. T cells were shown to be genetically restricted and to regulate different leukocyte populations, including B cells and professional phagocytes. The discovery of dendritic cells as major antigen-presenting cells and their surface expression of pattern recognition receptors revealed the mechanisms by which innate immunity instructs acquired immunity. Genetic analysis provided in-depth insights into the generation of antibody diversity by recombination, which in principle was shown to underlie diversity of the T cell receptor, as well. The invention of monoclonal antibodies not only provided ultimate proof for the unique antigen specificity of the antibody-producing plasma cell, it also paved the way for a new era of immunotherapy. Emil Behring demonstrated cure of infectious disease by serum therapy, illustrating how clinical studies can stimulate basic research. The recent discovery of checkpoint control for cancer therapy illustrates how clinical application benefits from insights into basic mechanisms. Last not least, perspectives on immunology progressed from a dichotomy between cellular-unspecific innate immunity and humoral-specific acquired immunity, toward the concept of complementary binarity.

Two signal half-century: From negative selection of self-reactivity to positive selection of near-self-reactivity

With the emergence of clonal selection ideas in the 1950s, the development of immune cell repertoires was seen to require the negative selection of self-reacting cells, with surviving cells exhibiting a broad range of specificities. Thus, confronting a universe of not-self-antigens, a potential host organism spread its resources widely. In the 1960s, the two signal hypothesis showed how this might work. However, in the 1970s an affinity/avidity model further proposed that anticipating a pathogen strategy of exploiting "holes" in the repertoire created by negative selection, hosts should also positively select near-self-reacting cells. A microbe mutating an antigen from a form foreign to its host to a form resembling that host should prevail over host defences with respect to that antigen. By mutating a step towards host self, along the path from non-self to self, it should come to dominate the microbe population. By progressive stepwise mutations, such microbes would become better adapted, to the detriment of their hosts. But they would lose this advantage if, as they mutated closer to host self, they encountered progressively stiffer host defences. Thus, as described in the affinity/avidity model, positive selection of lymphocytes for specificities that were very close to, but not quite, anti-self (ie, "anti-near-self") should be an important host adaptation. While positive selection affects both B and T cells, mechanisms are uncertain. Converging evidence from studies of lymphocyte activation, either polyclonally (with lectins as "antigen-analogs") or monoclonally (by specific antigen), supports the original generic affinity/avidity model for countering mutations towards host self.