Cellular immune responses have an important role in the immunologic rejection of vascularized tissue in animals and man24
. In mouse immunotherapy models, transfer of immune T lymphocytes but not antibodies protects mice from tumor challenge; elimination of endogenous CD8+
T cells abrogates both protective and therapeutic antitumor effects; and extensive T cell infiltrates are commonly seen in tumors and allografts undergoing immunologic rejection (reviewed in ref. 24
). The induction of CD8+
cells with specific immune reactivity can depend on interactions with other cell types such as CD4+
and antigen-presenting cells, although the final effector in most models is the CD8+
lymphocyte. Thus, the majority of cancer immunotherapy efforts are devoted to stimulating cellular immune responses against the growing tumor.
Three criteria are required for the immunologic destruction of established tumors: (i) sufficient numbers of immune cells with highly avid recognition of tumor antigens must be generated in vivo (ii) these cells must traffic to and infiltrate the tumor stroma, and (iii) the immune cells must be activated at the tumor site to manifest appropriate effector mechanisms such as direct lysis or cytokine secretion capable of causing tumor destruction.
Although immune T cells capable of recognizing tumor antigens can be generated by direct immunization in tumor-bearing mice, there are no cancer vaccine models that reproducibly demonstrate that vascularized tumors can be rejected by this approach. The rapid growth of extensively passaged mouse tumors that often express retroviruses represents an obstacle to the study of cancer vaccines that may require extensive immunizations over a long period of time. Thus, most mouse models of cancer vaccines assess the ability to prevent the outgrowth of tumor injected after vaccination or attempt to treat tumors a few days after transplantation when the tumors are not yet vascularized. The presence of even large numbers of immune T cells capable of recognizing tumor antigens in mice is insufficient to mediate tumor regression4, 25
. T cells must be in the correct state of activation and differentiation in order to mediate antitumor effects. This point is often underappreciated in the analysis of human immunotherapy trials.
In mice transgenic for T cell receptors that recognize tumor antigens, virtually all T lymphocytes can recognize tumor, but tumor growth and lethality are often unaffected. Inadequate numbers or avidity of the immune cells, the inability of the tumor to activate quiescent or precursor lymphocytes, tolerance mechanisms including anergy, and suppressor influences produced by the tumor or the immune system itself are among the mechanisms that can prevent tumor destruction by immune cells25, 26
. These obstacles must be overcome if cancer vaccines are to be effective in mediating cancer regression.
More encouraging, however, are studies that demonstrate the ability of adoptively transferred antitumor immune T cells to mediate the rejection of large vascularized tumors in mice under the appropriate conditions of host immune suppression and antigen stimulation. Large B16 melanomas can be rejected in mice after host lymphodepletion when antitumor T cells are transferred along with antigen-specific vaccination and IL-2 (ref. 4
). Cell transfer combined with vaccination, γc
cytokines and prior host immuno suppression all can maximize tumor destruction10, 11, 27
The success of these cell transfer approaches in mice has its counterpart in recent human clinical trials5
. In patients with metastatic melanoma refractory to treatment with high dose IL-2 and to chemotherapy, the transfer of in vitro–
activated and expanded autologous antitumor lymphocytes plus IL-2 into lymphodepleted patients mediated objective cancer regressions in 6 of 13 patients. Persistence of the transferred cells was seen for up to four months after cell administration5
. Patient entry into this protocol has now been expanded and we now have observed objective cancer regressions in 18 (51%) of 35 patients, many of whom have bulky disease (data not shown).
The effectiveness of cell transfer immunotherapy also serves to highlight many of the obstacles confronting vaccine therapy approaches and suggests possible means to overcome them. Cancer vaccines often result in low levels of circulating immune cells. Pox virus vaccines have been reported to increase circulating human antitumor antigen-reactive T cells from fewer than 1 in 200,000 to about 1 in 40,000 (refs. 28,29
). In some peptide vaccine trials, frequencies of over 1 in 200 antitumor cells can be generated, yet tumor regression is still not seen30
. The cells generated often have low avidity for tumor recognition. In contrast, antitumor T cells used for cell transfer, generated in vitro
from tumor infiltrating lymphocytes (TILs) or from peripheral blood lymphocytes, can be obtained in large numbers (up to 1 × 1011
) and can be selected in vitro
for highly avid recognition of tumor antigens31
. Transfer of these cells into lymphodepleted hosts can result in 5–75% of circulating CD8+
cells with antitumor activity5
. Cancer vaccines may need to generate these levels to be clinically effective.
An important reason that T cells generated by cancer vaccines may not destroy solid tumors is the inability of the immune cells to infiltrate and become activated after an encounter with tumor antigen in vivo
. In contrast to solid tumors, lymphoid tumors allow easier access to the circulation and often express costimulatory molecules that are required in the afferent phase of the immune response, but may also be involved in the activation of memory cells. This may explain why lymphoid tumors have been reported to be more clinically responsive to dendritic cell vaccines32
. Solid tumors do not express these costimulatory molecules or produce the inflammatory environment necessary to convert quiescent precursor lymphocytes into activated lymphocytes with the effector functions required for tumor eradication. In contrast, immune cells generated and activated ex vivo
can be infused in a highly activated state, already displaying the necessary lytic and cytokine-secreting activities required to mediate the destruction of even large solid tumor masses. A challenge to the application of cancer vaccines is the development of methods to not only generate long-term memory cells but also activate these antitumor cells, possibly by improving methods of stimulating antigen presenting cells with new adjuvants in vivo
or by creating an inflammatory environment at the tumor site to promote the homing of effector lymphocytes to the tumor.
Recent research has emphasized the importance of active suppressor mechanisms arising both from the tumor and from the immune system itself that can inhibit antitumor immune reactions in vivo33, 34, 35
. Perhaps the most important of these regulatory effects are mediated by CD4+
lymphocytes with the ability to suppress both the proliferation and effector functions of immune cells. A major advantage of cell transfer therapies is the ability to deplete host lymphocytes, including these regulatory cells, before cell transfer, and this preparation is critical to the success of many preclinical cell transfer immunotherapies. For cancer vaccines to be effective, it may require the elimination of these regulatory T cells, and although reagents to selectively eliminate these cells in vivo
are being developed, their clinical efficacy has yet to be established. Chemotherapy- or radiation-induced lymphodepletion can eliminate regulatory cells but cannot be used in conjunction with cancer vaccines because the needed effector cells are also eliminated.
Studies in mouse models have defined additional principles important for human application. Cancer vaccines may be of greatest value when administered as a specific antigenic stimulant to transferred T cells, especially under conditions when host lymphocytes are eliminated that compete with the transferred cells for γc homeostatic cytokines such as IL-7, IL-15 and IL-21. Elimination of the ‘cellular sinks’ for cytokines may enable antitumor T cells to be activated by these endogenous cytokines.