Maternal Antibodies in Autism Spectrum Disorder: Isolation and Specificity
The concept that in utero environment, specifically maternal antibodies can contribute to the development of Autism spectrum disorders (ASD) has been entertained for over a decade, but only recently specific antibodies have been identified. Studies, including our own, have shown that significantly more mothers of children with ASD have brain-reactive antibodies than unselected women of child bearing age or mothers of a typically develop child. The hypothesis is that these anti-brain antibodies exploit the natural mechanism of immune protection of the fetus, cross the placenta, and, at a time when the fetal brain is not protected by a competent blood brain barrier, they can perturb fetal brain development. Indeed, several studies showed that mice or monkeys exposed in utero to such antibodies demonstrate ‘ASD’-like phenotype.
We opted to generate brain-reactive monoclonal antibodies from mothers with an ASD child, to study their antigenic specificities, contribution to ASD pathogenesis and to device a protection strategy.
We generated anti-brain monoclonal antibodies by first separating memory (CD27+) B cells from the blood of women with a child with ASD previously shown to have brain-reactive antibodies. We then incubated CD27+ B cells with fetal human brain homogenate labeled with biotin, followed by fluorochrome tagged stretavidin. B cells bound to fetal human brain antigens were isolated. Single CD27+ B cells were then isolated and immunoglobulin heavy and light chain variable region genes were amplified by PCR and expressed in vitro. We determined antigenic specificity of brain reactive monoclonal antibodies using a human protein array and a cell based assay. We determined pathogenicity by analyzing brain and behavior of the offspring following in uterine exposure to a monoclonal antibody cloned from a mother of an ASD child.
Results: We have generated a panel of brain-reactive monoclonal antibodies. Three monoclonal antibodies from 3 different mothers were found to bind the extracellular domain of contactin-associated protein-like 2 (Caspr2). We assessed the pathogenic potential of one of those monoclonal antibodies, C6. We intravenously administered either non-brain reactive control antibody B1 or C6 to pregnant mice on Embryonic day (E)13.5. We demonstrate that male but not female mice exposed in utero to the C6 monoclonal antibody display abnormal cortical development at E15.5 with a thinner cortical plate and a reduced number of proliferating cells. At adulthood, the brain exhibits decreased dendritic complexity of excitatory neurons and reduced numbers of inhibitory neurons in the hippocampus, and the live offspring exhibit impairments in sociability, flexible learning, and repetitive behavior. We suggest that this effect might cause by antibody mediated early internalization of AMPA receptors. We further demonstrated that anti-Caspr2 antibodies are more frequent in women with brain-reactive serology and a child with ASD
We show in a mouse model that exposure in-utero to a monoclonal anti-brain reactive antibody isolated from a mother of an ASD child induces neurodevelopmental effects in the offspring that can be observed already during the embryonic stage. Currently we are designing a strategy to protect the developing fetus from the harmful effect of anti-Caspr2 antibodies.