Most tumor-associated antigens have been identified as normal gene products that are overexpressed, preferentially expressed, or reexpressed in cancer cells (1
). This has resulted in a new paradigm in tumor immunotherapy that has redirected efforts to break tolerance and induce autoimmune responses against such antigens (1
). Here, we demonstrate that lymphopenia-induced homeostatic T cell proliferation is a physiologic process by which effective antitumor autoimmunity can be elicited.
This investigation is based on the recent demonstration that self-MHC/peptide recognition is important for T cells to undergo homeostatic proliferation, a process that maintains the near constant size of the peripheral T cell pool (12
). Homeostatic adjustment of T cell numbers is necessary after lymphopenia has been induced by a variety of insults (infection, irradiation, or cytotoxic drugs), is polyclonal, and occurs without deliberate immunization.
An important question is whether T cells undergoing homeostatic proliferation can acquire effector function and thus cause autoimmunity. Several studies have shown that T cells driven to homeostatic proliferation acquired several, but not all, of the activation/memory phenotype markers associated with responses to foreign antigen (12
). Moreover, TCR-transgenic CD8+
cells that had undergone homeostatic proliferation were shown to kill target cells in vivo in a peptide- and TCR-dependent manner, and to express IFN-γ after stimulation with anti-CD3 (33
). Similarly, even polyclonal CD8+
cells that had undergone homeostatic proliferation rapidly secreted IFN-γ after in vitro anti-CD3 stimulation and killed ConA-coated syngeneic targets; control naive CD8+
cells were devoid of such activities (34
). We demonstrate here that polyclonal homeostatic proliferation leads to in vivo effector function against melanoma and, to some extent, colon carcinoma cells. Interestingly, this antimelanoma response was associated with vitiligo, also seen in other types of melanoma immunotherapies and considered to reflect therapeutic efficacy (5
). Our findings establish a fascinating link between self-recognition as a requirement for homeostatic T cell proliferation, antitumor responses, and autoimmune disease.
To undergo homeostatic expansion, T cells must enter certain areas in secondary lymphoid organs, such as periarteriolar lymphocyte sheaths in spleen and paracortex in LNs (20
). Inhibition of tumor growth was much less evident when normal T cells were transfused into LTα-deficient mice than into LTα+/+
mice. LTα-deficient mice lack LNs and have variable degrees of disorganized splenic architecture (29
). More importantly, transfusion of β7/CD62L-deficient T cells, which do not home into LNs (31
), but home and proliferate efficiently in the spleen, did not enhance the antitumor response seen in irradiated normal mice. Thus, the antitumor effect associated with the homeostatic expansion of transfused T cells in lymphopenic hosts occurs only if the T cells encounter the tumor antigen(s) in LNs. Our observations are compatible with those of Ochsenbein et al. (35
), who found that metastasizing tumors that failed to “seed” in LNs and spleen were “ignored” by the immune system. Additionally, no cytotoxic T cells could be induced in tumor cell–challenged alymphoplastic (Aly
) B6 mice that lack LNs, but possess lymphatic vessels, spleen, and T cells. Antigen presentation at this site may be mediated directly by the tumor cells, by trafficking of tumor antigen–loaded DCs from the tumor site to the draining lymph node, and/or indirectly through cross-priming (36
Remarkably, the antimelanoma effect was associated with long-term specific memory, since rechallenge with the same tumor cells resulted in rejection, while growth of an unrelated tumor (colon carcinoma) was unimpeded. Moreover, homeostatic T cell proliferation was effective even after the melanoma tumor had been established, without the need for tumor cell rechallenge. It is likely that, during the primary tumor cell injection, some of these cells or antigens thereof were transported to the draining LNs, but the response, if any, was ineffective due to the predominance of low-affinity T cells and/or low precursor frequency. In contrast, when the tumor antigens become available to the immune system at the initiation of homeostatic proliferation, selection and expansion of a few high-affinity cells led to strong effector function. Previous studies have shown that only a fraction (~30%) of T cells can undergo homeostatic proliferation (13
), and this fraction appears to encompass cells with higher self-affinity (38
Our findings clearly indicate that T cells can be induced to mount an effective autoimmune response against self-antigens when homeostatic expansion occurs at the time of antigen encounter. The expanded T cells should recognize a wide range of self-molecules, but this response was primarily tumor specific, as shown by cytotoxicity and IFN-γ induction upon incubation of the expanded cells with the relevant, but not irrelevant, tumor cells. This is probably because the tumor-associated self-antigens are highly enriched in regional LNs. This conclusion is supported by the findings of Oehen and Brduscha-Riem (33
), who showed that in vivo antiviral effector function by homeostatically expanding LCMV-specific transgenic T cells depended on the presence of the antigen at the early stages of expansion. The present findings are also in agreement with those of Asavaroengchai et al. (39
), who observed efficient antitumor responses in mice immunized with DCs pulsed with tumor cell lysates in the early phase of bone marrow reconstitution, wherein the lymphopenic environment leads to homeostasis-driven T cell proliferation. Preliminary findings by Hu et al. (40
) have also shown efficacy of homeostatic T cell proliferation in reducing the numbers of pulmonary metastases in mice challenged with a poorly immunogenic prostate tumor.
Several vaccination methods have been shown to overcome tolerance and elicit effective antitumor autoimmune responses, but these labor-intensive procedures are often based on the use of a single epitope that might not induce a broadly efficacious response. In contrast, the procedure described here relies on induction of lymphopenia in conjunction with whole tumor cell injection and transfusion of naive, unprimed, autologous T cells with a wide spectrum of self-affinities. This is simple and likely initiates a broad antitumor response without the a priori need to know the tumor antigen(s). Even when specific vaccines are used, it seems advantageous to couple them with simultaneous induction of T cell homeostatic proliferation, since it is likely to enhance the antitumor response and effectiveness. Furthermore, the present results suggest that, in clinical practice, tumor immunotherapy should commence immediately after completion of irradiation or cytotoxic therapies to take advantage of the lymphopenia and attendant homeostatic T cell expansion.