It is well-known that among mammals, somatic hypermutation (SHM) occurs primarily in germinal center B cells and is the driving force for antibody affinity maturation, since it introduces point mutations into the variable domains of immunoglobulin (IG) genes. The dgyw/wrch motif (where d=a or g or t, y=c or t, w=t or a, r=a or g, and h = c or t or a) has been found to be the principal hotspot for activation cytidine deaminase (AID)-induced g:u lesions in rearranged IG genes during SHM. The identification of the corresponding germline genes allowed us to find out for the first time in a mammalian organism, that γδ TR evolution has been favoured in the dromedary by mutation in the productively rearranged T cell receptor gamma (TRG) and delta (TRD) genes. Our data provide evidence that in the γδ T cell receptor of the dromedary, the conformation of the antigen-binding site may be altered by the SHM which generates mutations in the variable region at a frequency far beyond the rate of spontaneous mutations. Furthermore, analysis of clonal lineages suggests the occurrence of sustained and sequential mutational events and selection in both TRG and TRD cDNA clones. A more detailed analysis of clonal radiation in cDNA sequences showed that replacement (R) mutations in framework regions (FR-IMGT) appear to be early in the lineages. In addition, these R founder mutations are nonconservative of the amino acid physicochemical properties, i.e. most of them change non-hydrophobic amino acid residues to hydrophobic ones, implying that the selection for during their clonal expansion is favored by an increase in the structural intrinsic stability of the γδ receptor. As a consequence, the T cell clone is positively selected for with respect to both the survival and proliferation, thereby acquiring an advantage in growth, in comparison to all other clones. We can considere that: (i) the increased stability in the γδ heterodimer could influence the affinity maturation to a given antigen in a manner similar to that of immunoglobulin genes; (ii) alternatively, structural changes in the γδ heterodimer, that is stabilized by mutations in FR-IMGT in genealogical related clones, could enable the acquisition of new antigenic specificity. Therefore the purpose of SHM in dromedary TRG/TRD genes might also be to increase the repertoire diversity. The above considerations further confirm the claim that the Camelidae, until now considered a case of evolutionary innovation because of the heavy-chain antibodies, represent a new mammalian model particularly useful for understanding the role of γδ T cell receptor diversity in the immune system function.
Dromedary (Camelus dromedarius) T cell receptor gamma (TRG) and delta (TRD) chains repertoire
ANTONACCI, Rachele;Linguiti, G.;CICCARESE, Salvatrice Maria
2016-01-01
Abstract
It is well-known that among mammals, somatic hypermutation (SHM) occurs primarily in germinal center B cells and is the driving force for antibody affinity maturation, since it introduces point mutations into the variable domains of immunoglobulin (IG) genes. The dgyw/wrch motif (where d=a or g or t, y=c or t, w=t or a, r=a or g, and h = c or t or a) has been found to be the principal hotspot for activation cytidine deaminase (AID)-induced g:u lesions in rearranged IG genes during SHM. The identification of the corresponding germline genes allowed us to find out for the first time in a mammalian organism, that γδ TR evolution has been favoured in the dromedary by mutation in the productively rearranged T cell receptor gamma (TRG) and delta (TRD) genes. Our data provide evidence that in the γδ T cell receptor of the dromedary, the conformation of the antigen-binding site may be altered by the SHM which generates mutations in the variable region at a frequency far beyond the rate of spontaneous mutations. Furthermore, analysis of clonal lineages suggests the occurrence of sustained and sequential mutational events and selection in both TRG and TRD cDNA clones. A more detailed analysis of clonal radiation in cDNA sequences showed that replacement (R) mutations in framework regions (FR-IMGT) appear to be early in the lineages. In addition, these R founder mutations are nonconservative of the amino acid physicochemical properties, i.e. most of them change non-hydrophobic amino acid residues to hydrophobic ones, implying that the selection for during their clonal expansion is favored by an increase in the structural intrinsic stability of the γδ receptor. As a consequence, the T cell clone is positively selected for with respect to both the survival and proliferation, thereby acquiring an advantage in growth, in comparison to all other clones. We can considere that: (i) the increased stability in the γδ heterodimer could influence the affinity maturation to a given antigen in a manner similar to that of immunoglobulin genes; (ii) alternatively, structural changes in the γδ heterodimer, that is stabilized by mutations in FR-IMGT in genealogical related clones, could enable the acquisition of new antigenic specificity. Therefore the purpose of SHM in dromedary TRG/TRD genes might also be to increase the repertoire diversity. The above considerations further confirm the claim that the Camelidae, until now considered a case of evolutionary innovation because of the heavy-chain antibodies, represent a new mammalian model particularly useful for understanding the role of γδ T cell receptor diversity in the immune system function.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
