Agonist monoclonal antibodies (mAbs) to CD137, a co-stimulatory TNF receptor family member expressed on activated T and NK cells, can induce immune-mediated rejection of multiple murine tumor types, and a fully human anti-CD137 mAb, BMS-663513, is in early-phase clinical trials in solid tumors. Significant activity has been seen in murine lymphoma models, both alone and in combination with anti-CD20 mAbs, providing rationale for clinical studies in lymphoma patients. Recently, however, CD137 up-regulation on activated human B cells has been reported, with CD137 ligation causing enhanced B cell proliferation and survival. This raises the concern that mAb binding to CD137, if present, on B cell neoplasms may promote tumor cell proliferation and/or resistance to apoptosis that may counteract the beneficial effects on T and NK cells. We therefore sought to assess the expression of CD137 on a series of human cell lines and primary tumor samples from patients with B-cell neoplasms, and if expressed, to explore the consequences of ligation with the anti-CD137 agonist BMS-66513. First, archived paraffin-embedded lymph node specimens from patients with low-grade B-cell lymphoma (n=11: 5 follicular, 4 marginal zone, 2 small lymphocytic) and diffuse large B-cell lymphoma (n=15) were stained for CD137 by immunohistochemistry. Reactive tonsillar tissue served as a positive control. No CD137 expression was observed within any tumor cells. Next, fresh samples from 14 additional patients with known tumor involvement of peripheral blood or bone marrow (8 chronic lymphocytic leukemia, 1 mantle cell lymphoma, 3 myeloma, 2 marginal zone lymphoma) were analyzed by multi-color flow cytometry. Again, no CD137 expression was observed on the gated neoplastic cells. Baseline surface expression of CD137 was similarly absent in all B cell-derived lines tested (Raji, FCTxFL2, FSCCL, DoHH2, Jeko-1, RPMI8226). However, activation with PMA/Ionomycin could reproducibly induce CD137 expression (% positive: 0.17% → 91%) after 24 hours in 1 of the lines: the follicular lymphoma FSCCL. Interestingly, this was the only line tested that lacked constitutive expression of CD137 ligand (CD137L), suggesting some reciprocal regulation of ligand and receptor expression. Despite this up-regulation of CD137, in vitro ligation of PMA/Ionomycin-activated FSCCL cells with BMS-66513 did not further increase tumor cell proliferation, nor protect the cells from activation-induced cell death, in contrast to effects of CD137 ligation reported in normal B cells (Zhang et al, J Immunol 2010; 184:787). Similarly, BMS-663513 treatment of activated, CD137+ FSCCL cells did not diminish the apoptosis induced by doxorubicin or bortezomib treatment. In addition, FSCCL cells recovered from ascites 7 and 14 days following intraperitoneal injection in SCID mice did not express CD137, implying that CD137 up-regulation is not occurring in vivo during tumor growth. Finally, treatment of FSCCL cells with rituximab, either in vitro or in vivo, did not induce CD137 expression. In conclusion, we demonstrate a lack of steady-state CD137 expression on malignant B cells, confirming the prior study by Houot et al (Blood 2009; 114:3431) and extending these findings to include CLL/SLL for the first time. While CD137 could be induced in a single cell line upon non-specific activation, CD137 expression on FSCCL cells was not seen under physiologic conditions likely to be encountered in the clinical setting, consistent with the primary patient data. Furthermore, even when CD137 was expressed, ligation with the agonist anti-CD137 mAb BMS-663513 did not provide a pro-proliferative or anti-apoptotic signal. These studies provide reassurance and further rationale for exploring agonist anti-CD137 antibodies as therapies for B cell neoplasms.
Borghaei:Lilly, Genentech, Amgen, Pfizer: Honoraria, Research Funding. Jure-Kunkel:Bristol Meyers Squibb: Employment.
Asterisk with author names denotes non-ASH members.