NK- and CD8(+) T cell-mediated eradication of established tumors by peritumoral injection of CpG-containing oligodeoxynucleotides.

Unmethylated cytosine-phosphorothioate-guanine (CpG) containing oligodeoxynucleotides (CpG-ODN) are known to act as adjuvants and powerful activators of the innate immune system. We investigated the therapeutic effect of CpG-ODN on a variety of established mouse tumors including AG104A, IE7 fibrosarcoma, B16 melanoma, and 3LL lung carcinoma. These tumors are only weakly immunogenic and notoriously difficult to treat. Repeated peritumoral injection of CpG-ODN resulted in complete rejection or strong inhibition of tumor growth, whereas systemic application had only partial effects. The CpG-ODN-induced tumor rejection was found to be mediated by both NK and tumor-specific CD8(+) T cells. Comparison of parental tumors and variants rendered more antigenic by transfection with tumor Ags suggested that the efficiency of the CpG-ODN therapy correlated with the antigenicity of the tumors. Peritumoral CpG-ODN treatment was even effective in a situation where the immune system was tolerant for the tumor Ag, as shown by breakage of tolerance and tumor elimination. These results suggest that peritumoral application of CpG-ODN acts locally by inducing NK cells, and also leads to efficient presentation of tumor Ags and stimulation of CD8(+) effector and memory T cells, thus providing a powerful antitumor therapy that can be also applied without knowledge of the tumor Ag.

Autoaggression and tumor rejection: it takes more than self-specific T-cell activation.

Establishment of self-tolerance prevents autoaggression against organ-specific self-antigens. This beneficial effect, however, may in turn be responsible for tumor immune evasion. Thus, dissecting the mechanisms leading to the breakdown of self-tolerance in autoimmune diseases might provide insights for successful antitumor immune therapies. In a variety of animal models, organ- or tumor-specific immunity has been described, focusing on antigen-specific T-cell activation. Here, we discuss two transgenic mouse models which demonstrate that both autoaggression and tumor rejection require more than activated, self-reactive T cells. TCR transgenic mice, which are tolerant to a liver-specific MHC class I antigen, Kb, can be activated to reject Kb-positive grafts, but fail to attack Kb-expressing liver. However, autoaggression occurs when activated T cells are combined with "conditioning" of the target organ by irradiation or infection with a liver-specific pathogen. Similarly, in a mouse model of islet cell carcinoma, neither co-stimulatory tumor cells nor highly activated antitumor lymphocytes provoke an effective immune response against the tumor. Instead, a combination of activated lymphocytes and irradiation is required for lymphocyte infiltration into solid tumors. Both model systems provide evidence that although activated antigen-specific lymphocytes are a prerequisite for autoaggression, effector cell extravasation and appropriate interaction with the target organ/tumor are equally important. Thus, we propose that the organ/tumor microenvironment is a critical parameter in determining the effectiveness of an anti-self immune response.

A two-step model for the induction of organ-specific autoimmunity.

Peripheral tolerance is considered to be a safeguard against autoimmunity but the mere existence of anergic T cells renders them potentially dangerous. Using transgenic mice that were tolerant to a foreign MHC class I antigen (Kb) exclusively expressed in the liver, we investigated whether reversal of tolerance in vivo would directly result in autoimmunity. Breaking of tolerance was achieved by application of tumour cells expressing both Kb and interleukin 2. Despite the fact that the respective mice were now able to reject Kb-positive grafts, the reversed T cells did not infiltrate and attack the Kb-positive liver. However, when the liver was 'conditioned' through an inflammatory reaction either by irradiation or by infection with Listeria, massive T cell infiltration and liver damage were observed in the reversed mice. The results show that at least two steps are required for autoimmunity: (1) activation of antigen-specific T cells, and (2) conditioning of the target organ. It will be important to determine the factors leading to conditioning but it is likely that adhesion molecules are involved. These experiments are not only of relevance for treatment of autoimmune disease but also for tumour therapy.

Failure to induce organ-specific autoimmunity by breaking of tolerance: importance of the microenvironment.

Peripheral tolerance is considered to be a safeguard against autoimmunity. Using a TCR-transgenic mouse system displaying peripheral tolerance against a liver-specific MHC class I Kb antigen, we investigated whether the breaking of tolerance would result in autoimmunity. Reversal of tolerance was achieved by simultaneous challenge with cells expressing the Kb autoantigen and IL-2. Tolerance could not be broken with IL-2 alone or when Kb- and IL-2-expressing cells were applied to different sites of the mice. However, despite the presence of activated autoreactive T cells that were able to reject Kb-positive grafts no autoaggression against the Kb-positive liver was observed. These results indicate that breaking of tolerance per se is not sufficient to cause liver-specific autoimmunity. However, when in addition to breaking tolerance the mice were infected with a liver-specific pathogen, autoaggression occurred. Thus, in this system at least two independent steps seem to be required for organ-specific autoimmunity: reversal of peripheral tolerance resulting in functional activation of autoreactive T cells and conditioning of the liver microenvironment which enables the activated T cells to cause tissue damage.