Toll-like Receptor-7 Tolerizes Malignant B Cells and Enhances Killing by Cytotoxic Agents

Abstract

Chronic activation through Toll-like receptors (TLR) occurs in a number of pathologic settings, but has not been studied to the same extent as primary activation. TLR7, expressed by B cells and some dendritic cells, recognizes molecular patterns associated with viruses that can be mimicked by synthetic imidazoquinolines. In response to primary stimulation with the imidazoquinoline, S28690, human mononuclear cells produced tumor necrosis factor-A, but were unable to do so upon restimulation with S28690. This state of ''tolerization'' lasted at least 5 days. Using chronic lymphocytic leukemia B cells as a model to facilitate biochemical analysis, the tolerized state was found to be associated with altered receptor components, including down-regulated expression of TLR7 mRNA and decreased levels of interleukin-1 receptorassociated kinase 1. Tolerization was characterized by a transcriptionally regulated block in stress-activated protein kinase and nuclear factor KB activation, with relatively preserved activation of extracellular signal-regulated kinase (ERK). Tolerized chronic lymphocytic leukemia cells were found to be more sensitive to cytotoxic chemotherapeutic agents, in part through altered stress-activated protein kinase signaling pathways. This property of the TLR7-tolerized state may potentially be exploited in the treatment of B cell cancers.

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

“…The observations with CLL cells in vitro (Shi et al, 2007) suggest that induction of the tolerized state requires strong initial TLR signaling. Low doses of TLR agonists, which fail to cause strong JNK and NF-kB activation and high TNF-a production, do not tolerize.…”

“…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). This property makes TLR agonists particularly appealing, as they may be able to improve the activity of conventional chemotherapies, even when the patient's immune system is too weak to control cancer progression on its own.…”

“…However, 2 days after initial exposure, they no longer made TNF-a or activated the JNK or NF-kB stress signaling pathways in response to the agonist (Shi et al, 2007). These cells were said to have been 'tolerized' by analogy with TLR-activated normal monocytes (Fan and Cook, 2004).…”

“…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.…”

“…Accordingly, our understanding of the TLR-tolerized state (defined only as the inability to make TNF-a upon restimulation with a TLR agonist; Fan and Cook, 2004) is incomplete. The state seems to result from a cell differentiation process mediated by genes whose expression can be blocked by the transcriptional inhibitor, actinomycin D (Shi et al, 2007). The nature of the genes required is not clear, but may include the cytokines IL-10 and TGF-b (as these have been implicated in the endotoxin tolerant state in monocytes; Randow et al, 1995) along with intracellular molecules, such as SOCS1, IL receptor-associated kinase M, MyD88s, ST2 and A20, which have been implicated in negative regulation of TLR signaling (Miggin and O'Neill, 2006).…”