Autism Spectrum Disorders (ASD) are heterogeneous neurodevelopmental disorders characterized by deficits in social communication as well as by repetitive patterns of behaviors and interests1. In recent decades, a striking increase in ASD prevalence has occurred, with 1/68 children diagnosed with ASD in the United States2. A number of factors are likely to have contributed to this rise, including improvements in diagnosis and epidemiological strategies as well as lifestyle changes, including exposure to environmental insults, new dietary habits and increased medication use3. Such data highlights the urgent need for a deeper understanding of the underlying pathophysiological processes in ASD, and thereby to innovative therapeutic strategies. Clarification of the underpinnings of gene and environment interactions represents a major theoretical and clinical target.
ASD is now widely accepted as being highly heritable4, with environmental risk factors interacting with genetic background. The development of ASD involves not only alterations in brain functioning and maturation, but also changes in immune processes. Alterations in immune functioning in ASD have been repeatedly demonstrated (for review, see5) showing, at least in subgroups of ASD patients: (i) a deleterious effect of prenatal or perinatal infectious pathogen exposure6; (ii) a pro-inflammatory state7 often concomitant with abnormal cell-mediated immunity8 or inflammatory-mediated gut dysbiosis9,10; (iii) a frequent autoimmune component observed in mothers of ASD off-spring as well as in ASD patients, with circulating anti-brain autoantibodies sometimes correlating with disease severity and/or behavior impairments11,12,13,14,15,16.
Such data clearly indicate a role for immune alterations in ASD, including interactions of the innate and adaptive immune systems. Previous research has reported relationships between the genetic diversity of loci encoding major immune molecules and ASD risk, including evidence of genetically-driven innate immune alterations in ASD7,17,18,19,20,21,22,23,24. Although the time scale for their interactions requires clarification, the co-existence of infections, inflammation and auto-immunity in ASD indicates that the foremost genetic susceptibility candidate(s) may be expected to lie in the vicinity of the highly polymorphic Human leukocyte antigen (HLA) super-locus25.
Hosted by the major histocompatibility complex (MHC) region on the short arm of the chromosome 6 (6p21.3–22.1), the HLA cluster is characterized by the highest polymorphism rate of the human genome26 (IMGT/HLA Database; http://www.ebi.ac.uk/imgt/hla). The cell surface HLA molecules, upon interactions with T cell receptors (TCRs), drive specific adaptive immune responses through antigen processing and presentation. While the HLA -A, -B and -C molecules encoded by the classical HLA class I gene cluster govern cellular immune processes, their class II counterparts (HLA-DRB1, -DQB1 and -DPB1, encoded by genes located in the HLA-class II sub-region) are crucial in mediating humoral immune responses. HLA molecules are also essential in a wide array of other physiological processes, including brain development and homeostasis27,28. Consequently, HLA genetic diversity has been intensively investigated in disease-association studies29, especially in regard to infectious, inflammatory and autoimmune disorders25,29,30,31.
In ASD, the influence of class I or II HLA polymorphisms has been explored, without providing any clear substantial evidence of a possible HLA-related pathway. This may be the consequence of the relatively small sample sizes used, the frequent use of non-molecular HLA genotyping techniques, and the scarcity of analysis at haplotype levels32,33,34,35,36,37,38,39,40,41,42,43,44,45,46. However, the polygenic contribution to the risk of several severe psychiatric disorders, including ASD, was confirmed in a meta-analysis of genome wide association studies (GWAS) where the MHC was the most significant shared risk loci47, reinforcing the notion that MHC-linked impairments in immune regulation may constitute a common risk factor across these disorders. However, despite the importance of these findings, it still requires clarification as to the precise involvement of any HLA locus in ASD, at least in part due to the imperfect coverage of HLA diversity in GWAS.
In order to provide additional information as to the role of HLA in ASD, we undertook a case-control study to more precisely explore the polymorphisms of the HLA class II loci, at allele, genotype and haplotype levels, using standardized molecular techniques.