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.

The innate ability of B lymphoma cells to escape control by tumor-reactive T cells must be overcome to develop effective immunotherapies for these diseases. Because signals from both the innate and adaptive immune systems direct the acquisition of strong immunogenicity by professional APCs, the effects of IL-2 and the TLR-7 agonist, S28690, on the immunogenic properties of chronic lymphocytic leukemia (CLL) B cells were studied. IL-2 with S28690 caused CLL cells to proliferate and increased their expression of B7-family members, production of TNF-α and IL-10, and levels of tyrosine-phosphorylated STAT-1 and STAT-3 proteins. S28690 increased CD25 expression on CLL cells and sensitized them to IL-2 signaling. However, IL-2 did not change TLR-7 expression or signaling in CLL cells. The ability to stimulate T cell proliferation required additional activation of protein kinase C, which inhibited tumor cell proliferation, “switched off” IL-10 production, and caused essentially all CLL cells (regardless of clinical stage) to acquire a CD83highCD80highCD86highCD54high surface phenotype marked by the activation of STAT-1 without STAT-3. These findings suggest that TLR-7 “licenses” human B cells to respond to cytokines of the adaptive immune system (such as IL-2) and provide a strategy to increase the immunogenicity of lymphoma cells for therapeutic purposes.

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.