The γ-aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the brain but also acts as an immunomodulatory molecule in the blood (1). GABA is synthesized from glutamate via the enzyme glutamic acid decarboxylase (GAD, two isoforms 65 and 67). GAD65 is the major autoantigen in type 1 diabetes (T1D), however, it is still not known how the level of GABA changes in the blood and if it affects the cytokine release and proliferation of immune cells in T1D. Nondiabetic (ND) individuals (n=30) and T1D patients (n=64) were included in the study. Plasma, peripheral blood mononuclear cells (PBMCs) and CD3+ T cells were isolated from fresh blood samples and CD4+ T cells from buffy coats. The GABA concentration and 92 cytokines were measured in plasma from the subjects by GABA ELISA and multiplex Proximity Extension Assay (PEA) assays, respectively. Twenty-six cytokines were significantly increased in plasma from T1D patients as compared to ND individuals. The plasma GABA concentration was significantly higher in T1D patients (mean ± SEM, ND: 501±32 nM; T1D: 649±42 nM, two-tailed Student’s t-test, p<0.05) and correlated with levels of 10 cytokines in plasma. The transcriptome analysis of isolated CD3+ T cells using RNA-seq revealed that the expression of 6 genes involved in the cholesterol biosynthesis pathway was significantly reduced in T1D samples. The expression levels of 2 out of these 6 genes (MSMO1 and CYP51A1) were negatively correlated with plasma GABA concentration (Spearman rank correlation, p<0.05). Isolated PBMCs and CD4+ T cells were stimulated with anti-CD3 antibody to examine the effect of GABA on cell proliferation and cytokine secretion in the culture media. GABA (100 nM) significantly inhibited proliferation of PBMCs from T1D patients but not ND individuals. In addition, GABA (100 nM) decreased the release of 47 and 16 cytokines in the medium from stimulated PBMCs of T1D patients and ND individuals, respectively. Interestingly, stimulated CD4+ T cells from ND individuals can be grouped into GABA responder and non-responder T cells based on the response to GABA (100 and 500 nM) in the proliferation assay. In the non-responder T cells, GABA only regulated the release of 8 cytokines. In contrast, in the responder T cells, GABA decreased cell proliferation and release of 37 cytokines. These results suggest the immunomodulatory function of GABA is concentration-dependent, varies among immune cell subpopulations and is altered in T1D (2).