RESUMO
Infant Abs induced by viruses exhibit poor functional activity compared with those of adults. The human B cell response to rotavirus is dominated by use of the V(H)1-46 gene segment in both adults and infants, but only adult sequences are highly mutated. We investigated in detail the kinetic, structural, and functional advantage conferred by individual naturally occurring somatic mutations in rotavirus-specific human Abs encoded by the immunodominant V(H)1-46 gene segment. Adult Abs achieved enhanced binding through naturally occurring somatic mutations in the H chain CDR2 region that conferred a markedly prolonged off-rate and a desirable increase in antiviral potency. Three-dimensional cryoelectron microscopy studies of Ag-Ab complexes revealed the mechanism of viral inhibition to be the binding of high-affinity Abs at the viral RNA release pore in the double-layer particle. These structure-function studies suggest a molecular basis for the poor quality of Abs made in infancy following virus infection or immunization.
Assuntos
Anticorpos Antivirais/fisiologia , Imunoglobulina D/biossíntese , Fragmentos Fab das Imunoglobulinas/fisiologia , Rotavirus/imunologia , Anticorpos Antivirais/biossíntese , Anticorpos Antivirais/genética , Anticorpos Antivirais/ultraestrutura , Antígenos Virais/imunologia , Antígenos Virais/metabolismo , Sítios de Ligação de Anticorpos/genética , Proteínas do Capsídeo/imunologia , Proteínas do Capsídeo/metabolismo , Humanos , Epitopos Imunodominantes/metabolismo , Imunoglobulina D/genética , Imunoglobulina D/fisiologia , Imunoglobulina D/ultraestrutura , Fragmentos Fab das Imunoglobulinas/biossíntese , Fragmentos Fab das Imunoglobulinas/genética , Fragmentos Fab das Imunoglobulinas/ultraestrutura , Cadeias Pesadas de Imunoglobulinas/biossíntese , Cadeias Pesadas de Imunoglobulinas/genética , Cadeias Pesadas de Imunoglobulinas/ultraestrutura , Região Variável de Imunoglobulina/biossíntese , Região Variável de Imunoglobulina/genética , Região Variável de Imunoglobulina/ultraestrutura , Cinética , Vírus Sinciciais Respiratórios/genética , Vírus Sinciciais Respiratórios/imunologia , Rotavirus/genética , Hipermutação Somática de ImunoglobulinaRESUMO
Mature, naive B cells coexpress IgD and IgM with identical binding sites. In this study, the binding properties of such IgM and IgD are compared to determine how size and shape may influence their ability to bind Ag and thus function as receptors. To dissect their intrinsic binding properties, recombinant IgM and IgD were produced in soluble form as monomers of the basic H(2)L(2) Ab architecture, each with two Ag binding sites. Since these sites are connected with a hinge region in IgD and structural Ig domains in IgM, the two molecules differ significantly in this region. The results show that IgD exhibited the larger angle and longer distance between its binding sites, as well as having the greater flexibility. Relative functional affinity was assessed on two antigenic surfaces with high or low epitope density, respectively. At high epitope density, IgM had a higher functional affinity for the Ag compared with IgD. The order was reversed at low epitope density due to a decrease in the functional affinity of IgM. Studies of binding kinetics showed similar association rates for both molecules. The dissociation rate, however, was slower for IgM at high epitope density and for IgD at low epitope density. Taken together, the results show that IgM and IgD with identical Ag binding regions have different Ag binding properties.
