Endogenous T Cell Receptors Drive Non-Specific Suppression by Foxp3 MHC Class I-Restricted T Cell Receptor-Transduced Polyclonal CD4+ T Cells in Models of Graft-Versus-Host Disease.

Abstract 3152

Alloreactive immune responses directed against malignant cells in recipients of allogeneic hematopoietic stem cell transplants are able to cure patients with hematological cancers. However, such immune responses may cause severe morbidity when directed against healthy recipient tissue, resulting in graft-versus-host disease (GvHD).

Naturally occurring regulatory T cells (Tregs) are CD4+ T cells characterized by their expression of the transcription factor Foxp3. Whilst adoptively transferred polyclonal Tregs suppress GvHD in several murine models, their lack of specificity may compromise beneficial immunity against malignancy or infection. The generation of MHC class I-restricted, alloantigen-specific Tregs would allow them to recognize antigen presented directly on GvHD target tissues, concentrating their sites of activation at these tissues and possibly reducing the potential for non-specific immune suppression.

We have generated ‘converted’ Tregs through retroviral transfer of genes encoding Foxp3 and specific MHC class I-restricted T cell receptors (TCRs) into polyclonal conventional CD4+ T cells. We used the 2C-TCR, which recognizes the MHC class I molecule H-2L^d, expressed in Balb/c and other H-2^d mice, in complex with the ubiquitously expressed peptide p2Ca; and the MH-TCR, which recognizes the MHC class I molecule H-2D^b, expressed in B6 and other H-2^b mice, in complex with the male peptide WMHHNMDLI.

In vitro, Foxp3 2C-TCR-transduced B6 polyclonal CD4+ T cells were hyporesponsive to stimulation and suppressed the alloreactive proliferative response of B6 CD4+ and CD8+ T cells to Balb/c splenocytes, consistent with the acquisition of regulatory function. When adoptively transferred to lethally irradiated Balb/c recipients of MHC-mismatched B6 bone marrow and conventional T cells, Foxp3 2C-TCR-transduced B6 polyclonal CD4+ T cells significantly reduced early proliferation of donor T cells, weight loss and GvHD score in the recipients. Similarly, polyclonal CD4+ T cells transduced with Foxp3 and the MH-TCR caused marked suppression of allogeneic responses both in vitro and in vivo. However, while both the 2C-TCR and the MH-TCR conferred specificity to their cognate antigens in vitro, the potent suppression in these in vivo models was independent of the cognate antigen for the transduced TCRs. This non-specific suppression was markedly reduced when class I-restricted TCRs were transduced into OT-II Rag1^-/- CD4+ T cells that are transgenic for a single endogenous TCR.

These findings demonstrate an important role for the endogenous TCRs in driving non-specific suppression by polyclonal CD4+ T cells transduced with Foxp3 and class I-restricted TCRs, and suggest that strategies to downregulate endogenous TCRs will be required to achieve antigen-specific suppression in TCR gene-modified regulatory T cells.