Critical evaluation of regulatory T cells in autoimmunity: are the most potent regulatory specificities being ignored?

Abstract

The identification of CD4+ CD25+ Foxp3+ regulatory T (Treg) cells as natural regulators of immunity in the periphery and tissues has stimulated tremendous interest in developing therapeutic strategies for autoimmune diseases. In this review, the site of origin, antigen specificity, homing markers and cytokine profiles of Treg cells were evaluated in autoimmune colitis and type 1 diabetes, two examples in which Treg cells were effective as therapy. These studies were compared with studies of Treg cells in experimental autoimmune encephalomyelitis and multiple sclerosis, where successful therapy has not yet been achieved. Antigen-specific Treg cells appear to have more potent activity than polyclonal Treg cells and therefore hold more promise as therapeutic agents. However, Treg cells specific for the pathogenic T effector cells themselves have largely been overlooked and deserve consideration in future studies.

Figure 1

Frequencies of T-cell receptor (TCR) peptide-reactive peripheral blood mononuclear cells (PBMC) from TCR tripeptide-vaccinated patients with multiple sclerosis (MS). Reactivity to the tripeptide mixture of CDR2 peptides from BV5S2, BV6S5 and BV13S1 was assessed in seven subjects with MS by proliferation responses using the limiting dilution assay and by secretion of interleukin-10 (IL-10) or interferon-γ (IFN-γ) using the enzyme-linked immunosorbent spot-forming (ELISPOT) assay. Blood samples were collected from the subjects before vaccination and at weeks 8/9, 12, 24 and 48 of the trial and cultured as described in ref. 59. Data are presented as mean frequency ± SD for subjects with MS at entry, the maximum post-vaccination response, and at exit from the trial. ELISPOT frequencies represent the sum of responses to each of the three vaccinating TCR CDR2 peptides. Figure reproduced from a paper by Vandenbark AA, Culbertson NE, Bartholomew RM et al. (Therapeutic vaccination with a trivalent T cell receptor peptide vaccine restores deficient FoxP3 expression and TCR recognition in subjects with multiple sclerosis) in Immunology 2007; 123: 66–78, with the permission of Wiley-Blackwell.

Figure 2

T-cell receptor (TCR) vaccination restores FoxP3 message and protein expression. Peripheral blood mononuclear cells (PBMC) were collected from patients with multiple sclerosis (MS) before vaccination and at the indicated time-points during treatment with TCR tripeptides, as well as from age-matched and gender-matched healthy control (HC) donors. The cells were sorted into CD4⁺ CD25⁺ and CD4⁺ CD25⁻ populations, and messenger RNA was extracted and evaluated for expression of Foxp3 and HPRT-1 genes. Sorted cells from the same subjects were also evaluated for Foxp3 and HPRT-1 protein by Western blots. (a) Note reduced Foxp3 expression in the CD4⁺ CD25⁺ cells from subjects with MS versus HC donors before vaccination, and significantly enhanced expression to levels even higher than HC donors on week 12 and in all subsequent weeks tested during the vaccination procedure. (b) Foxp3 expression was much lower in the CD4⁺ CD25⁻ population and was not different between MS and HC donors before vaccination, but was significantly enhanced to levels higher than HC donors in the subjects with MS during vaccination with TCR tripeptides. (c) Foxp3 protein levels were reduced in CD4⁺ CD25⁺ T cells from subjects with MS versus HC donors before vaccination, but were restored to levels higher than in HC donors in the MS subjects during vaccination. Figure reproduced from a paper by Vandenbark AA, Culbertson NE, Bartholomew RM et al. (Therapeutic vaccination with a trivalent T cell receptor peptide vaccine restores deficient FoxP3 expression and TCR recognition in subjects with multiple sclerosis) in Immunology 2007; 123: 66–78, with the permission of Wiley-Blackwell.

Figure 3

Developmental pathways of T-cell receptor (TCR)-specific regulatory T (Treg) cells. TCR-reactive CD4⁺ CD25⁺ Foxp3⁺ regulatory T cells are generated intrathymically in response to naturally processed and presented TCR determinants expressed on activated positively selected T cells; or are taken up, processed and presented from dead or dying T cells undergoing negative selection by thymic antigen-presenting cells (APC). Treg cells seed the periphery and enter a pool of CD4⁺ CD25⁺ Foxp3⁺ T cells (mostly naïve cells) that can be potentiated after contact with an activated effector T (Teff) cells expressing the cognate V gene or by TCR peptides introduced by vaccination to undergo expansion. Activated CD4⁺ CD25⁺ FoxP3⁺ Treg cells remain in the circulation or enter central nervous system tissues through the blood–brain barrier where they may be reactivated specifically by direct interaction with T helper type 1 (Th1) or Th17 cells bearing the cognate TCR. These cells acquire Treg activity that mediates cell-contact-dependent inhibition of proliferation and cytokine release by target T cells expressing the cognate TCR, or bystander T cells of the same or different myelin antigen specificity expressing a different TCR. Figure modified from that published in Vandenbark.