Myeloid glycosylation defects lead to a spontaneous common variable immunodeficiency-like condition with associated hemolytic anemia and antilymphocyte autoimmunity

Immune dysfunction in MGAT2-CDG: A clinical report and review of the literature

Glycosylation is a critical post/peri-translational modification required for the appropriate development and function of the immune system. As an example, abnormalities in glycosylation can cause antibody deficiency and reduced lymphocyte signaling, although the phenotype can be complex given the diverse roles of glycosylation. Human MGAT2 encodes N-acetylglucosaminyltransferase II, which is a critical enzyme in the processing of oligomannose to complex N-glycans. Complex N-glycans are essential for immune system functionality, but only one individual with MGAT2-CDG has been described to have an abnormal immunologic evaluation. MGAT2-CDG (CDG-IIa) is a congenital disorder of glycosylation (CDG) associated with profound global developmental disability, hypotonia, early onset epilepsy, and other multisystem manifestations. Here, we report a 4-year old female with MGAT2-CDG due to a novel homozygous pathogenic variant in MGAT2, a 4-base pair deletion, c.1006_1009delGACA. In addition to clinical features previously described in MGAT2-CDG, she experienced episodic asystole, persistent hypogammaglobulinemia, and defective ex vivo mitogen and antigen proliferative responses, but intact specific vaccine antibody titers. Her infection history has been mild despite the testing abnormalities. We compare this patient to the 15 previously reported patients in the literature, thus expanding both the genotypic and phenotypic spectrum for MGAT2-CDG.

CDG and immune response: From bedside to bench and back

Glycosylation is an essential biological process that adds structural and functional diversity to cells and molecules, participating in physiological processes such as immunity. The immune response is driven and modulated by protein-attached glycans that mediate cell-cell interactions, pathogen recognition and cell activation. Therefore, abnormal glycosylation can be associated with deranged immune responses. Within human diseases presenting immunological defects are congenital disorders of glycosylation (CDG), a family of around 130 rare and complex genetic diseases. In this review, we have identified 23 CDG with immunological involvement, characterized by an increased propensity to-often life-threatening-infection. Inflammatory and autoimmune complications were found in 7 CDG types. CDG natural history(ies) and the mechanisms behind the immunological anomalies are still poorly understood. However, in some cases, alterations in pathogen recognition and intracellular signaling (eg, TGF-β1, NFAT, and NF-κB) have been suggested. Targeted therapies to restore immune defects are only available for PGM3-CDG and SLC35C1-CDG. Fostering research on glycoimmunology may elucidate the involved pathophysiological mechanisms and open new therapeutic avenues, thus improving CDG patients' quality of life.

Dendritic cell-specific Mgat2 knockout mice show antigen presentation defects but reveal an unexpected CD11c expression pattern

Zwitterionic polysaccharide antigens such as polysaccharide A (PSA) from Bacteroides fragilis have been shown to activate CD4⁺ T cells upon presentation by class II major histocompatibility complex (MHCII) on professional antigen presenting cells. For T cell recognition and activation, high affinity binding between MHCII and PSA is required, and complex N-glycans on conserved MHCII asparagine residues play a central role in controlling this interaction. By truncating these glycans in a myeloid-specific knockout of Mgat2, created using the LyzM-CRE mouse (M-cKO), we previously reported defects in PSA responses in vivo. Unfortunately, the M-cKO also showed a propensity to develop common variable immunodeficiency with autoimmune hemolytic anemia features. Here, we describe a novel murine model in which Mgat2 was targeted for ablation using the dendritic cell (DC)-specific CD11c-CRE-GFP strain in order to develop a more specific and robust in vivo model of PSA presentation defects (DC-cKO). This study shows that Mgat2 deficient DCs from DC-cKO mice show ablation of PSA presentation and downstream T cell activation in vitro. However, the CD11c promoter was unexpectedly active and triggered Mgat2 deletion within multiple hematopoietic lineages, showed remarkably poor penetrance within native DC populations, and produced almost undetectable levels of green fluorescent protein signal. These findings show that the CD11c promoter is not DC-specific, and extreme care should be taken in the interpretation of data using any mouse created using the CD11c-CRE model.