Immunomodulatory effects of Toll-like receptor-7 activation on chronic lymphocytic leukemia cells

Abstract

Weak immunogenicity of chronic lymphocytic leukemia (CLL) cells may contribute to disease progression and inhibit effective immunotherapy. Accordingly, agents that enhance the immunogenicity of CLL cells may be useful in immunotherapeutic approaches to this disease. Since Toll-like receptors (TLRs) are major regulators of innate immunity and initiation of adaptive immunity, we studied the effects of viral pathogen associated molecular pattern agonists (that are recognized by TLRs) on the costimulatory phenotype and function of CLL cells. CLL cells (especially those with high endogenous expression of CD38) responded to TLR7-activating imidazoquinolines and guanosine analogs by increasing costimulatory molecule expression, producing inflammatory cytokines, and becoming more sensitive to killing by cytotoxic effectors. Additional activation of protein kinase C pathways increased the ability to stimulate T-cell proliferation, blocked phosphorylation of the transcription factor, signal transducer and activator of transcription (STAT)3, and resulted in the acquisition of a dendritic cell surface phenotype by TLR7-activated CLL cells. Normal B cells also responded to TLR7 activation by increasing costimulatory molecule expression and cytokine production. These findings suggest a potential role for TLR7 agonists in CLL immunotherapy.

Toll‐like receptors on B‐CLL cells: expression and functional consequences of their stimulation

“…It has been suggested that the stimulation of TLRs expressed on B-CLL cells could increase immunogenicity of tumor cells and thus potentially contribute to the induction of leukemia-specific immune response. [15][16][17][18] Several studies also suggested that TLR-9 agonists decrease viability of B-CLL cells and increase susceptibility of B-CLL cells to apoptosis in the cell culture. 19,20 However, a concise study including the analysis of TLRs expression pattern, comparison with normal B cells and functional consequences of TLRs stimulation on B-CLL cells has not yet been performed.…”

Obstacles to effective Toll-like receptor agonist therapy for hematologic malignancies

“…However, recent phase I/II trials of CpG oligonucleotides in lymphoma patients provided no evidence of clinical responses despite increased NK activation (Link et al, 2006). Similarly, our recent phase I/II trial of a specific TLR7 agonist (852A; Harrison et al, 2007) in CLL patients has not shown strong clinical activity (manuscript in preparation), in contrast to the striking effects that were noted in vitro (Spaner et al, 2006;Shi et al, 2007).…”

“…Accordingly, it seems more likely that the therapeutic effects of BCG or Coley's toxins resulted from the direct effects of TLR agonists on tumor cells. Importantly, primary leukemia cells become more sensitive to CTLs after several days of activation by TLR agonists in vitro (Spaner et al, 2006), a finding that may be highly relevant when thinking of using such agents to treat hematologic cancers. The ability of TLR agonists to sensitize leukemia cells to killing by CTLs appears to extend to cytotoxic agents (Shi et al, 2007).…”

“…These cells were said to have been 'tolerized' by analogy with TLR-activated normal monocytes (Fan and Cook, 2004). Importantly, CLL cells in the TLR-tolerized state became exquisitely sensitive to CTLs (Spaner et al, 2006) and chemotherapeutic agents (Shi et al, 2007) in vitro.…”

“…It is possible that the initial strong TLR signaling drives the cells into cycle, making them sensitive to drugs that act on proliferating cells, such as microtubule inhibitors (Goodman et al, 1994). Extrinsic death pathways may also be opened in these cells, as FAS (CD95) levels increase on TLR-activated CLL cells (Spaner et al, 2006). While a complete understanding of many aspects of the TLR-tolerized state is lacking, it does seem clear that a strong initial response to the TLR agonist, producing high levels of TNF-a, and supported by energy generated from oxidative phosphorylation, is minimally required to achieve the state in vitro, and presumably in vivo.…”