Insights into the genetics and biochemistry of signaling adaptor modules and nk cell receptors from study of primary immunodeficiency

Zusammenfassung

Many immune cell receptors are modular in design; the ligand binding function and signaling capacity are separated into two different elements. One common signaling adaptor molecule expressed in both Natural Killer and T cells is CD247. The opportunity to study immune cells from a CD247 deficient patient let us analyze the effects of the lack of this adaptor module on human lymphocyte biology. In the absence of CD247, T cell receptor assembly was compromised and its expression at the cell surface markedly diminished, abrogating antigen-specific immune responses by the patient and increasing susceptibility to infections. With regard to NK cells, surface expression of CD247-coupled receptors and their function after specific ligation was compromised. Moreover, as well as these direct effects, the absence of functional T cells also impaired NK cell development and proper maturation indirectly. Consistent with prior studies on CD247-deficient patients, it was possible to detect revertant T and NK cells in this particular patient. Subsequent genetic analyses of the patients T cells, after brief in vitro expansion, revealed two classes of revertant cells: a true back mutation and a compensatory mutation, both compatible with restored CD247 protein expression and T cell receptor assembly. This reversion phenomenon has been reported for only a set of genes affected in PIDs. Further informatics analysis of the genetic variation in other PID genes where reversion has been reported showed a higher rate of missense variants in the coding sequence of these genes compared to PID affected genes with no reversion cases or control genes, leading to the hypothesis that the occurrence of somatic mutations that reconstitute genetic defects in PID is related to an increased propensity of those genes to mutate. To date, the interactions that mediate CD247 assembly with counterpart receptors depend on contacts between single transmembrane residues of opposing charge. However, CD16A does not contain a positively charged residue in its transmembrane domain. Detailed biochemical study of CD16A receptor complex formation revealed a novel mode of assembly based on multiple polar and aromatic interactions that could be extended to other Fc receptors, CD64 and FcR1. Altogether, our results illustrate how the human immune system adapts, escapes by reversion of genetic defects and the biochemical plasticity of CD247 signaling adaptor module demonstrated by the ability to couple many receptors by different mechanisms.