Requirements for effective antitumor responses of TCR transduced T cells.

Adoptive transfer of TCR gene-modified T cells has been proposed as an attractive approach to target tumors for which it is difficult or impossible to induce strong tumor-specific T cell responses by vaccination. Whereas the feasibility of generating tumor Ag-specific T cells by gene transfer has been demonstrated, the factors that determine the in vivo effectiveness of TCR-modified T cells are largely unknown. We have analyzed the value of a number of clinically feasible strategies to enhance the antitumor potential of TCR modified T cells. These experiments reveal three factors that contribute greatly to the in vivo potency of TCR-modified T cells. First, irradiation-induced host conditioning is superior to vaccine-induced activation of genetically modified T cells. Second, increasing TCR expression through genetic optimization of TCR sequences has a profound effect on in vivo antitumor activity. Third, a high precursor frequency of TCR modified T cells within the graft is essential. Tumors that ultimately progress in animals treated with this optimized regimen for TCR-based adoptive cell transfer invariably display a reduced expression of the target Ag. This suggests TCR gene therapy can achieve a sufficiently strong selective pressure to warrant the simultaneous targeting of multiple Ags. The strategies outlined in this study should be of value to enhance the antitumor activity of TCR-modified T cells in clinical trials.

TCR gene therapy of spontaneous prostate carcinoma requires in vivo T cell activation.

Analogous to the clinical use of recombinant high-affinity Abs, transfer of TCR genes may be used to create a T cell compartment specific for self-Ags to which the endogenous T cell repertoire is immune tolerant. In this study, we show in a spontaneous prostate carcinoma model that the combination of vaccination with adoptive transfer of small numbers of T cells that are genetically modified with a tumor-specific TCR results in a marked suppression of tumor development, even though both treatments are by themselves without effect. These results demonstrate the value of TCR gene transfer to target otherwise nonimmunogenic tumor-associated self-Ags provided that adoptive transfer occurs under conditions that allow in vivo expansion of the TCR-modified T cells.

In vivo antigen stability affects DNA vaccine immunogenicity.

The factors that determine the immunogenicity of Ags encoded by viral vaccines or DNA vaccines in vivo are largely unknown. Depending on whether T cell induction occurs via direct presentation of vaccine-encoded epitopes or via one of the different proposed pathways for Ag cross-presentation, the effect of intracellular Ag stability on immunogenicity may possibly vary. However, the influence of Ag stability on CD8(+) T cell induction has not been addressed in clinically relevant vaccine models, nor has the accumulation of vaccine-encoded Ags been monitored in vivo. In this study, we describe the relationship between in vivo Ag stability and immunogenicity of DNA vaccine-encoded Ags. We show that in vivo accumulation of DNA vaccine-encoded Ags is required for the efficient induction of CD8(+) T cell responses. These data suggest that many of the currently used transgene designs in DNA vaccination trials may be suboptimal, and that one should either use pathogen-derived or tumor-associated Ags that are intrinsically stable, or should increase the stability of vaccine-encoded Ags by genetic engineering.

Targeting self-antigens through allogeneic TCR gene transfer.

Adoptive transfer of T-cell receptor (TCR) genes has been proposed as an attractive approach for immunotherapy in cases where the endogenous T-cell repertoire is insufficient. While there are promising data demonstrating the capacity of TCR-modified T cells to react to foreign antigen encounter, the feasibility of targeting tumor-associated self-antigens has not been addressed. Here we demonstrate that T-cell receptor gene transfer allows the induction of defined self-antigen-specific T-cell responses, even when the endogenous T-cell repertoire is nonreactive. Furthermore, we show that adoptive transfer of T-cell receptor genes can be used to induce strong antigen-specific T-cell responsiveness in partially MHC-mismatched hosts without detectable graft versus host disease. These results demonstrate the feasibility of using a collection of "off the shelf" T-cell receptor genes to target defined tumor-associated self-antigens and thereby form a clear incentive to test this immunotherapeutic approach in a clinical setting.

Generation of T cell help through a MHC class I-restricted TCR.

CD4+ T cells that are activated by a MHC class II/peptide encounter can induce maturation of APCs and promote cytotoxic CD8+ T cell responses. Unfortunately, the number of well-defined tumor-specific CD4+ T cell epitopes that can be exploited for adoptive immunotherapy is limited. To determine whether Th cell responses can be generated by redirecting CD4+ T cells to MHC class I ligands, we have introduced MHC class I-restricted TCRs into postthymic murine CD4+ T cells and examined CD4+ T cell activation and helper function in vitro and in vivo. These experiments indicate that Ag-specific CD4+ T cell help can be induced by the engagement of MHC class I-restricted TCRs in peripheral CD4+ T cells but that it is highly dependent on the coreceptor function of the CD8beta-chain. The ability to generate Th cell immunity by infusion of MHC class I-restricted Th cells may prove useful for the induction of tumor-specific T cell immunity in cases where MHC class II-associated epitopes are lacking.