How Do CD4(+) T Cells Detect and Eliminate Tumor Cells That Either Lack or Express MHC Class II Molecules?

Abstract

CD4(+) T cells contribute to tumor eradication, even in the absence of CD8(+) T cells. Cytotoxic CD4(+) T cells can directly kill MHC class II positive tumor cells. More surprisingly, CD4(+) T cells can indirectly eliminate tumor cells that lack MHC class II expression. Here, we review the mechanisms of direct and indirect CD4(+) T cell-mediated elimination of tumor cells. An emphasis is put on T cell receptor (TCR) transgenic models, where anti-tumor responses of naïve CD4(+) T cells of defined specificity can be tracked. Some generalizations can tentatively be made. For both MHCII(POS) and MHCII(NEG) tumors, presentation of tumor-specific antigen by host antigen-presenting cells (APCs) appears to be required for CD4(+) T cell priming. This has been extensively studied in a myeloma model (MOPC315), where host APCs in tumor-draining lymph nodes are primed with secreted tumor antigen. Upon antigen recognition, naïve CD4(+) T cells differentiate into Th1 cells and migrate to the tumor. At the tumor site, the mechanisms for elimination of MHCII(POS) and MHCII(NEG) tumor cells differ. In a TCR-transgenic B16 melanoma model, MHCII(POS) melanoma cells are directly killed by cytotoxic CD4(+) T cells in a perforin/granzyme B-dependent manner. By contrast, MHCII(NEG) myeloma cells are killed by IFN-γ stimulated M1-like macrophages. In summary, while the priming phase of CD4(+) T cells appears similar for MHCII(POS) and MHCII(NEG) tumors, the killing mechanisms are different. Unresolved issues and directions for future research are addressed.

Keywords: CD4+ T cells; MHC class II; T cell receptor transgenic; T helper 1; multiple myeloma; transgenic mouse models; tumor antigen; tumor immunology.

Figure 1

Direct and indirect killing of tumor cells by CD4⁺ T cells. (A) CD8⁺ T cells can directly kill tumor cells that express MHC class I molecules, whereas (B) cytotoxic CD4⁺ T cells can kill tumor cells that express MHC class II molecules. (C) While most tumor types express MHC class I molecules, they often lack expression of MHC class II. How do CD4⁺ T cells recognize and eliminate MHCII^NEG tumor cells? (D) CD4⁺ T cells may kill MHC class II negative (MHC II^NEG) tumors by a mechanism where (i) tumor antigen secreted by tumor cells is processed and presented by MHCII^POS macrophages to CD4⁺ T cells. (ii) Bi-directional interaction/activation of macrophages and CD4⁺ T cells (iii) activates tumoricidal macrophages that in turn kill the tumor cells (In addition, activated CD4⁺ T cells themselves could possibly directly kill tumor cell in a TCR/MHC II-independent manner.).

Figure 2

The MOPC315 myeloma model. Naïve tumor-specific CD4⁺ T cells protect against MHC II^NEG tumor challenge in the absence of other T cells and B cells. (A) MOPC315 myeloma cells of BALB/c origin secrete an IgA M315 myeloma protein with a mutated λ2 light chain referred to as λ2³¹⁵. M315 is endocytosed and processed by BALB/c APCs, and a CDR3 sequence that includes residues 91–101 of λ2³¹⁵ is presented on the MHC class II molecule I-E^d to Id-specific CD4⁺ T cells. The peptide that is recognized by Id-specific CD4⁺ T cells contains somatic mutations in positions 94, 95, and 96 (45, 79, 80). Based on the αβ TCR of the Id-recognizing 4B2A1 clone, a TCR-transgenic mouse was generated (46). Most CD4⁺ T cells in this mouse express a transgenic TCR that can be tracked by a clonotype-specific mAb [Nomenclature: antigenic determinants in immunoglobulin variable (V) regions are called idiotopes (Id). The 91–101 peptide is thus an Id-peptide, and the CD4⁺ T cells that recognize this Id-peptide presented by I-E^d are called Id-specific]. (B) Id-specific TCR-transgenic mice on an immunosufficient background (BALB/c) are resistant to a challenge with Id^POS MOPC315 cells but succumb to Id^NEG J558 myeloma cells [reproduced with permission from Proc Natl Acad Sci (26), Copyright 1994 National Academy of Sciences, U.S.A.]. (C). Id-specific TCR-transgenic mice on an immunodeficient background (SCID), lacking other T and B cells than Id-specific CD4⁺ T cells, are also resistant to MOPC315 tumor development [reproduced with permission from Immunity (34)]. Tumor resistance could be transferred with purified Id-specific CD4⁺ T cells to SCID mice (27).

Figure 3

The Matrigel assay. A novel approach to unravel the dynamics of CD4⁺ T cell-mediated primary anti-tumor immune responses. At day 0, subcutaneous injections with MOPC315 tumor cells suspended in liquid Matrigel. When the Matrigel solution reaches body temperature, it jellifies and forms a plug containing the tumor cells. At various time points after injection (n days), the Matrigel plug and tumor-draining lymph nodes are dissected out and analyzed ex vivo for cellular content, function of cells, and cytokines (34, 36, 39, 85).

Figure 4

Mechanism of rejection of MHCII^NEG myeloma cells by Id-specific CD4⁺ T cells. The following events are based on experiments where Id-secreting MOPC315 suspended in liquid Matrigel was injected subcutaneously in TCR-transgenic mice. (i–viii). (i) At the incipient tumor site, macrophages [CD11b⁺, CD11c⁻, CD80/CD86⁺ MHC II^LO, F4/80⁺] start to infiltrate the tumor/Matrigel from day +1. Tumor-infiltrating macrophages become Id-primed by extracellular myeloma protein by the conventional MHC II presentation pathway (65). (ii) Extracellular Id+ myeloma protein (or possibly Id-primed tumor APCs) drain to lymph nodes where Id-primed APCs stimulate Id-specific CD4⁺ T cells. Uncertainties as to the mechanism of Id+ Ag draining and the identity of Id-primed APCs are indicated by a question mark (?). (iii) Id-specific CD4⁺ T cells become activated by day +3, are substantially expanded by day +6 (34), and polarize into Th1 cells by day +8 (39, 85). Upon activation in the tumor-draining lymph node, a number of molecules are significantly upregulated on the surface of the Id-specific CD4⁺ T cells, while some are consistently downregulated (85). (iv) Activated CD4⁺ T cells (CD62L^LOW) leave the lymph node and accumulate at the tumor site from day +6 (34, 86). (v) At the incipient tumor site, infiltrating Id-specific CD4⁺ T cells are re-activated by Id-primed macrophages (34). (vi) Moreover, in addition to a sustained Th1 phenotype, the tumor-infiltrating CD4⁺ T cells dramatically change expression of a number of surface molecules (85). Several molecules are upregulated on both activated CD4⁺ T cells in the tumor-draining lymph node, and on tumor-infiltrating CD4⁺ T cells, although at higher levels in the latter population. (vii) IFN-γ produced by tumor-infiltrating Th1 cells activates macrophages that up-regulate MHC class II on the cell surface and show increased expression of M1-associated surface molecules (34, 39). IFN-γ-activated macrophages acquire a tumoricidal phenotype with the upregulation of cytotoxicity-associated markers including granzyme A/B, and NKG2D (39). In addition, purified activated macrophages can directly inhibit myeloma growth in vitro (34, 36, 39). The mechanisms underlying M1 macrophage-mediated growth inhibition is unknown, but once the macrophages are activated the growth inhibition is antigen independent (36). (viii) Analysis by gene expression profiling and Luminex multiplex cytokine analyses has revealed that the Id-specific CD4⁺ Th1-mediated anti-tumor immune response has a striking resemblance to the characteristics of acute inflammatory responses (39). Thus, we propose that Th1-mediated inflammatory responses may protect against cancer (87).