The presence of autoreactive CD4+ T cells in the peripheral blood of healthy human subjects was investigated after removal of CD4+CD25+ regulatory T cells (Treg). CD4+ T cells that were directed against the type 1 diabetes-associated autoantigen glutamic acid decarboxylase 65, the melanocyte differentiation Ag tyrosinase, and the cancer/testis tumor Ag NY-ESO-1 were readily derived from PBMC of healthy individuals. These autoreactive T cells could be visualized, using Ag-specific class II tetramer reagents, in the peripheral blood of most individuals examined. Addition of CD4+CD25+ Treg back to the CD4+CD25− population suppressed the expansion of the autoreactive T cells. Autoreactive T cells were cloned based on tetramer binding, and expressed characteristic activation markers upon self-Ag stimulation. These results show that autoreactive T cells are present in most healthy individuals and that Treg likely play an important role of keeping these autoreactive T cells in check.

The memory T cell response is polyclonal, with the magnitude and specificity of the response controlled in part by the burst size of T cells expanded from effector/memory precursors. Sensitive assays using HLA class II multimers were used to detect low-frequency Ag-specific T cells directed against influenza viral Ags in subjects immunized with the influenza vaccine. Direct ex vivo tetramer staining of PBMC from five individuals identified frequencies of hemagglutinin (HA) 306–318 tetramer binding CD4+ T cells in the peripheral blood ranging from 1 in 600 to 1 in 30,000 CD4+ T cells. These frequencies were validated by counting CFSElow, tetramer-positive T cells after in vitro expansion. Low frequency of T cells directed to other influenza epitopes, including DRA1*0101/DRB1*0401-restricted matrix protein 60–73, DRA1*0101/DRB1*0101-restricted matrix protein 18–29, DRA1*0101/DRB1*0701-restricted HA 232–244 and DRA1*0101/DRB1*0101-restricted nucleoprotein 206–217 were also determined. T cells which occurred at a frequency as low as 1 in 350,000 could be ascertained by in vitro expansion of precursors. Peripheral HA306–318-responsive T cells expanded 2- to 5-fold following influenza vaccination. Examination of phenotypic markers of the HA306–318-responsive T cells in the peripheral blood indicated that the majority were CD45RA−, CD27+, CD25−, CD28+, and CD62L−, while T cell clones derived from this population were CD45RA−, CD27−, CD25+, CD28+, and CD62L−.

Islet-specific glucose-6-phosphatase catalytic subunit-related protein (IGRP) is recognized as a major autoantigen for autoimmune type 1 diabetes (T1D) in the NOD mouse model. This study was undertaken to examine CD4+ T cell responses toward IGRP in human subjects. The tetramer-guided epitope mapping approach was used to identify IGRP-specific CD4+ T cell epitopes. IGRP23–35 and IGRP247–259 were identified as DRA1*0101/DRB1*0401-restricted epitopes. IGRP13–25 and IGRP226–238 were identified as DRA1*0101/DRB1*0301-restricted epitopes. IGRP-specific tetramers were used to evaluate the prevalence of IGRP-reactive T cells in healthy and T1D subjects. More than 80% of subjects with either DRB1*0401 or DRB1*0301 haplotype have IGRP-specific CD4+ T cell responses for at least one IGRP epitope. IGRP-specific T cells from both healthy and T1D groups produce both γ-IFN and IL-10. DRA1*0101/DRB1*0401 IGRP247–259-restricted T cells also show cross-reactivity to an epitope derived from liver/kidney glucose-6-phosphatase. The detection of IGRP-reactive T cells in both type 1 diabetic subjects and healthy subjects and recent reports of other autoreactive T cells detected in healthy subjects underscore the prevalence of potentially autoreactive T cells in the peripheral immune system of the general population.

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