Anti-CD19 directed chimeric antigen receptor (CAR) T-cell therapy has yielded high response rates and durable remissions in a subset of patients with B cell non-Hodgkin lymphoma (NHL); however, patients either progress or relapse after this therapy. Resistance is not well understood but may be related to loss of the target tumor antigen or to aspects of the tumor microenvironment. Defining resistance mechanisms will allow for enhancement of this therapy for lymphoma patients.
Mutations in HVEM (herpes virus entry mediator; TNFRSF14) are among the most frequent mutations in germinal center lymphomas. Dr. Michael Boice and colleagues investigate the function of HVEM on follicular lymphoma (FL) lymphomagenesis. Under normal circumstances, HVEM binds to the B and T lymphocyte attenuator (BTLA) receptor and inhibits the B cell. Loss of HVEM, through somatic mutations, and loss of BTLA, through inactivation of the histone methyltransferase KMT2D, appear to be mutually exclusive events that are seen in 20 to 30 percent and more than 50 percent of FL, respectively. Furthermore, this incidence does not increase with disease transformation, suggesting that these events occur early in oncogenesis. To assess the role of HVEM and BTLA in the development of FL, Dr. Boice and colleagues introduced short hairpin RNAs (shRNAs) against Hvem or empty vector controls into vavPBcl2 hematopoietic progenitor cells (HPCs) and injected them into lethally irradiated mice. Knockdown of Hvem resulted in a significant acceleration in lymphoma development, as well as increased phosphorylation of SYK, BTK, and BLNK, evidence of activation of the B cell receptor (BCR) pathway. Knockdown of Btla in the same FL model system yielded similar results.
The microenvironment is important for both the development and persistence of FL. Follicular dendritic cells (FDCs), fibroblast reticular cells (FRCs) and follicular T helper cells (TFH) are all known to support the malignant FL cell. Compared to a group of control lymphomas, lymphoma cell lines that are deficient in HVEM produced significantly more TNFα, LTα, and LTβ, which are essential activators of FDCs and FRCs. These levels are decreased upon treatment with soluble HVEM (solHVEM). Evaluation of the tumor microenvironment in HVEM-deficient tumors (both derived from cell lines and from primary FL tumors) revealed enhanced FDC networks and FRC activation, as well as increased levels of CXCL13, which is known to be produced by FDCs and FRCs and to attract TFH. Indeed these tumors were associated with increased numbers of TFH compared with control lymphomas, and with increased levels of TFH-produced cytokines.
SolHVEM, therefore, may be able to rescue HVEM-deficient, BTLA-intact B-cells and inhibit growth of HVEM-deficient, BTLA-intact lymphoma. To test this, Dr. Boice and colleagues treated the Bcl1 B-cell lymphoma cell line with solHVEM or ibrutinib after stimulation of the BCR with IgM. Both solHVEM and ibrutinib had similar inhibitory effects on BTK phosphorylation following BCR stimulation compared with controls, but this effect of solHVEM required intact BTLA. Similar results were seen in BTLA-expressing primary FL cell lines, regardless of whether the cells express HVEM or not. SolHVEM caused growth inhibition and induced apoptosis of BTLA-positive lymphoma cell lines in vitro as well as in vivo in mouse xenograft models. Finally, they treated xenografted DoHH2 lymphomas, which express BTLA but carry a homozygous HVEM deletion, with anti-CD19 CAR T-cells as well as anti-CD19 CARs engineered to express solHVEM. The latter resulted in enhanced cell killing.
In Brief
The identification of targetable and recurrent genetic mutations in Hvem in FL that lead to enhanced B cell activation and survival through both BCR activation and through the maintenance of a permissive microenvironment has therapeutic potential for a subset of FL patients. SolHVEM alone is unlikely to yield clinical benefit given both the difficulty in delivering the peptide drug to the lymphoma, and the genetic and immunologic complexity of the disease. Combining this therapy with CAR T cells takes advantage of the powerful anti-lymphoma effect of CAR T-cells and their ability to deliver the drug directly to the target. This is an elegant way to enhance CAR T cell therapies while targeting additional vulnerabilities of the lymphoma cell and its environment. Unlike CAR T cells directed against the nearly universal B cell antigen CD19, these modified CAR T-cells will only be of additional benefit to a genetically defined subgroup of patients. This system, however, can serve as a prototype for lymphomas with additional or alternative targets.
Competing Interests
Dr. Jacobson indicated no relevant conflicts of interest.
