W49 - Brain-homing Monocytes Identified in Pediatric Acute-onset Neuropsychiatric Syndrome (PANS) are Immunosuppressive

Pediatric Acute-onset Neuropsychiatric Syndrome (PANS) is characterized by an abrupt onset of obsessive-compulsive disorder and other psychiatric and somatic symptoms.  We found elevated levels of circulating monocytes in PANS patients compared to healthy controls. Human monocytes in blood can be divided into 3 subsets, which can differentiate to tissue-homing macrophages and dendritic cells during inflammation. We compared the immunophenotypes of peripheral monocytes in PANS patients (n = 18) and controls (n = 10) to determine their pro- vs anti-inflammatory immunophenotypes. Using flow cytometry, we found an increased level of pro-inflammatory monocyte-derived DC in PANS flare vs healthy control. We also observed an increase in M1-macrophage-type cells at flare and M2-type cells at improved state. Additionally, we identified a brain-homing monocyte subset that was significantly reduced during PANS flare, with a subsequent increase when clinical features of PANS improved. We found these cells in the cerebrospinal fluid of new-onset, but not chronic, PANS patients, confirming their brain-homing and BBB-crossing potential. Interestingly, we found that PANS plasma induced the expressions of these “brain-homing” markers in healthy monocytes.  Cluster distribution from scRNA-seq of myeloid cells revealed two distinct subsets that expressed our pre-defined brain-homing markers and a pseudotime trajectory analysis argued that these cells derive from CD14⁺ monocytes. Gene-set enrichment analysis showed that the CCR2^hi brain-homing subset expresses immunosuppressive genes, consistent with an anti-inflammatory role. The CCR2^lo subset expresses neuroprotective genes. Together, our data support a model in which distinct myeloid cell subsets contribute to PANS brain inflammation and its suppression.