Cytolytic helper T cells: a new addition to myeloma immunity Free

In this issue of Blood, Kim et al analyze the CD4⁺ T-cell compartment in the bone marrow of individuals with multiple myeloma and uncover the presence of a subset of helper T cells with the capacity to recognize and kill autologous cancer cells.¹ 

Recent years have brought significant progress in understanding the bone marrow immune microenvironment in multiple myeloma. Novel RNA sequencing technologies allowed the entire bone marrow immune compartment to be analyzed at single-cell resolution, revealing changes in major cell populations, including CD8 T cells and myeloid cells.^2,3 The inherent drawback of these approaches is that the necessary resolution to detect alterations in less frequent populations of cells is rapidly lost. The premise for the current study by Kim and coworkers was the recognition of an association between increased abundance of total CD4 T-helper cells and improved outcome of transplant-ineligible individuals newly diagnosed with myeloma. As bone marrow helper CD4 T cells are less numerous than their cytotoxic CD8 T-cell counterparts,⁴ knowledge on the nature of these T cells in relation to myeloma pathogenesis was limited.

Based on this initial finding, the team set out to characterize bone marrow CD4 T cells at high resolution by performing single-cell RNA sequencing on prepurified cells. The samples analyzed included patients with newly diagnosed myeloma, age-matched controls, and individuals with the precursor stages of myeloma: monoclonal gammopathy of undetermined significance (MGUS) and smoldering multiple myeloma (SMM).

The resulting data sets are a rich resource of data on T-helper cell populations across the plasma cell malignancy spectrum that will, without doubt, be a source of inspiration and future discovery. Interestingly, the association between improved outcome and T-helper cells was driven by a subpopulation of helper T cells with a T helper 1 (Th1)-like profile and expression of genes associated with cytotoxicity and terminal differentiation. This cytolytic population was enriched in patients with myeloma compared with controls and extending the analyses to include flow cytometry confirmed the increased presence of these cytolytic CD4 T cells in MGUS, SMM, and multiple myeloma in both bone marrow and peripheral blood.

Th1 cells are major histocompatibility complex (MHC) class II–restricted CD4⁺ T cells classically involved in antibacterial and antiviral immune responses. Nonetheless, as early as the 1980s, the ability of Th1 cells to acquire cytolytic capacity was recognized, mostly associated with chronic antigen stimulation.⁵ Since that time, cytolytic Th1 cells have been implicated in viral immunity and in immune responses to certain tumors.^6,7 

In the current study by Kim and colleagues, the presence of cytolytic T-helper cells is now documented in multiple myeloma. The association with multiple myeloma is significant as the paucity of the immune responses is a hallmark of full-blown myeloma,² and identifying immune components with the capacity to lyse myeloma cells could open up novel therapeutic opportunities.

Identifying what a T cell recognizes is key to understanding its functional contribution. The authors take advantage of the fact that each T-cell clone expresses a unique T-cell receptor (TCR), allowing enumeration of individual T-cell clones. Cytolytic T-helper cell clones expand during myeloma progression, suggesting active recognition of antigens. By comparing the sequences of the TCR to public databases, recognition of major viral epitopes was excluded. Conversely, functional assays with myeloma-derived peptides induced activation of cytolytic T-helper cells in approximately 25% of patients with myeloma tested. Together, these findings build a strong case for direct recognition of myeloma antigens by cytolytic T-helper cells.

In the final set of experiments, the mechanisms of myeloma cell lysis by T-helper cells were dissected. Among CD4 T-cell subpopulations, cytolytic T-helper cells uniquely express NKG2D, and expression levels of NKG2D are regulated by TCR signaling and cytokine stimulation. Expression of stress-induced ligands for the c-type lectin-like receptor NKG2D on myeloma cells has prognostic implications,⁸ and neutralization of NKG2D reduced the ability of patient-derived cytolytic T-helper cells to kill myeloma cells in coculture experiments.

To what extent cytolytic T-helper cells contribute to antimyeloma responses in vivo and to what extent are they active during different stages of the disease are difficult-to-answer questions. In general, cancer progression is associated with the inability of the immune system to control the tumor. Whether this holds true for both CD8 and CD4 cytolytic responses is still largely unresolved. The improved outcomes in transplant-ineligible patients who present with increased numbers of cytolytic T-helper cells is interesting, as it might suggest either a continuous T-helper cell-driven antitumor immune response or a therapy-induced reactivation of such a response in a subgroup of patients.

The current study provides novel insights into an understudied T-cell population in multiple myeloma and underscores the need for a better grasp on various aspects of antimyeloma immunity. Convincing evidence is presented for the recognition of multiple myeloma cell-derived peptide-MHC class II complexes by CD4 T cells and the subsequent generation of a Th1 response. This Th1-driven response will eventually lead to the generation of cytolytic Th1 cells. In myeloma, as in most cancers, MHC class I–restricted cytotoxic CD8 T-cell responses have been the focus of most studies. As a result, our understanding of T-helper responses is much less mature, and this study underscores the necessity of closing that gap. Th1 responses, in general driven by interleukin-12 and leading to interferon gamma and tumor necrosis factor-α production, activate macrophages and other myeloid cells, resulting in the production of inflammatory mediators and macrophage-mediated tumor-cell killing. The results presented in the current study warrant a reevaluation of such immune functionalities in multiple myeloma and highlight the need to include cytolytic T-helper cells as a relevant population in future studies.

Conflict-of-interest disclosure: T.C. declares no competing financial interests.