This review shows how tumor antigen cross-presentation is affected by the major therapeutic modalities including chemotherapy, radiotherapy, and surgery. We argue that this process could affect the way that a tumor works as its own cellular vaccine, and that it is differentially modulated by the choice of treatment. (Cancer Res 2006; 66(2): 601-4) It is estimated that tumors typically accumulate 10 12 or more mutations (1). Even though many of these were already present in the tissue cells from which the tumor originated and only a small proportion occur in the open reading frames of genes expressed by tumor cells, it is inevitable that any given cancer will express at least a few new antigenic determinants that could be recognized by the immune system. Despite this, the immune system rarely stops established tumor growth. It is likely that the same tolerance mechanisms that prevent immune responses against normal tissue cells expressing random mutations also block antitumor responses. Similar to self-antigens, tumor neo-antigens are constitutively and efficiently offered to the immune system through a process known as cross-presentation (2, 3). The fact that tumor neo-antigens are likely to be highly individuated suggests that the most promising approaches to immunotherapy will depend on our ability to turn a tumor into its own cellular vaccine (4).
What Is Cross-Presentation?Most cells, including many tumor cells, express MHC class I molecules and can present antigenic peptides to T cells. However, only professional antigen-presenting cells (APC), such as dendritic cells (DC) and macrophages, have the capacity to prime immune responses. These cells acquire antigen from donor cells such as tumor cells, then present the captured antigens via their own MHC class I molecules to CD8 T cells in a process known as antigen crosspresentation (3). However, the precise mechanisms are still controversial (5), and, importantly, the relevance of tumor antigen cross-presentation for the induction of antitumor responses has been questioned (6). In this review, we will discuss the molecular basis for cross-presentation, which we judge to be a constitutive but limited process and describe the basis of the subsequent immune response; cross-priming, when a response is measurable versus cross-tolerance, when it is not. We will argue that crosspresentation of tumor antigens is highly relevant for tumor immunotherapy but that the transition from cross-presentation to crosspriming needs help, either from CD4 T helper cells or via targeted immunotherapy. The combination of chemotherapy with its capacity to alter the rate of cross-presentation with immunotherapy may therefore present an excellent opportunity to achieve this.
Which Antigens Are Cross-Presented?The process of antigen cross-presentation was discovered in 1976, when it was shown that immunization with lymphoid cells congenic for minor histocompatibility antigens (7) or with allogeneic or semisyngeneic SV40-transformed cells (8) resulted in the generation of spec...