Antibodies Targeting Subunit 1 of the N-Methyl-D-Aspartate Receptor May Contribute to Immune Mediated Thrombocytopenia.

N-methyl-D-aspartate receptors (NMDARs) are essential for neuronal functioning in the central nervous system (CNS). They are also expressed on mammalian megakaryocytes and platelets. In contrast to the wealth of information about NMDARs on neurons, little is known about receptor expression and function outside the CNS. Signaling downstream of NMDAR appears to be important for the production of megakaryocytes and platelets in humans, with receptor antagonism leading to megakaryocytic dysplasia in vitro. In systemic lupus erythematosus (SLE), there is some evidence that anti-NMDAR antibodies reactive against subunits 2A/2B contribute to neuropsychiatric manifestations and possibly thrombocytopenia. Here, we hypothesized that anti-NMDAR antibodies may also contribute to immune mediated thrombocytopenia in hematological disorders, and conducted experiments to explore this possibility. Subjects with the following characteristics were tested by ELISA for the presence of serum antibodies reactive with extracellular domains of NMDAR subunit 1 (NR1), the essential receptor component that predominates on megakaryocytes and platelets: idiopathic thrombocytopenic purpura (ITP, n = 12, 5 males, mean age of 56 ± 6 yrs), myelodysplasia (MDS, n = 16, 8 males, mean age of 72 ± 4 yrs) and healthy blood donor controls (n = 45, 22 males, mean age of 52 ± 1.5 yrs). Sera of 5/12 ITP patients reacted with the NR1 protein, 3 of whom had a platelet count less than 30 × 10⁹/L. Two of the 5 positive ITP patients also had antinuclear antibodies (ANA), without meeting criteria for SLE. Unexpectedly, of the 16 patients with MDS, sera from 11 were reactive with the NR1 protein; these included 4 with refractory anemia, 4 with chronic myelomonocytic leukemia, and 3 with refractory anemia with excess of blasts. Four of this group were known to have other auto-antibodies (ANA, rheumatoid factor) or positive direct anti-globulin test. In 3 of these patients, platelet counts had previously improved with steroid therapy. At the time of testing, the mean platelet count in the group of NR1-reactive MDS patients was 39 ± 9 × 10⁹/L. One of 45 healthy blood donors showed reactivity against NR1. CNS disease was present in 2 of the total 16 NR1-reactive patients; one with ITP (epilepsy) and one with MDS (a history of a subarachnoid hemorrhage with residual hemiparesis and dysphasia). Any link between the generation of anti-NR1 antibodies and neurological disease manifestations in these patients is unknown. This study demonstrates that antibodies reactive with the NR1 subunit of NMDARs exist in some patients with ITP and MDS, suggesting a new mechanism for the induction of thrombocytopenia in these disorders. In addition, our results support work of others highlighting co-existent autoimmune processes in MDS. Further examination of the role of NMDAR-reactive antibodies in hematological disease is warranted.