Background: Chronic lymphocytic leukemia (CLL) cells are weakly immunogenic, a property that may contribute to disease progression and inhibit the effectiveness of immunotherapies such as vaccines. Low surface expression of co-stimulatory molecules contributes to this poor immunogenicity. CLL cells express Toll-Like-Receptor-7 (TLR7), a powerful modulator of innate immunity. TLR7 agonists may be capable of enhancing the immunogenicity of CLL cells and thereby increasing T cell mediated killing of CLL.

Methods: Circulating CLL cells were isolated directly from consenting patients by negative selection. TLR7 mRNA expression by CLL cells was demonstrated by RT-PCR. CLL cells were then incubated with S28690 (a TLR7 agonist), or with a negative control for 24–72h. Expression of the costimulatory molecules CD80, 83, 86, and 54 was determined by flow cytometry pre and post-incubation. Experiments were repeated in the presence of a NFkB inhibitor (dexamethasone), a p38 MAPK inhibitor, and a protein kinase C agonist (PDB). The effects of S28690 on phosphorylated-IkB and phosphorylated-STAT3 levels were measured by immunoblotting. The capacity of S28690-incubated CLL cells to stimulate T cell proliferation and killing was determined in mixed lymphocyte responses.

Results: All tested CLL samples (n=20) expressed TLR7 mRNA, while Jurkat cells (T cell origin) did not. After incubation with S28690, CD80, 83, 86, and 54 surface expression increased on all CLL samples tested. The relative increase varied from 4 to 9-fold and was positively correlated with CD38 expression. NF-kB and p38 inhibitors decreased the effects of S28690 on co-stimulatory molecule expression while PDB amplified the effect. After incubation with S28690, IkB and STAT3 phosphorylation increased in CLL cells. S28690-incubated-CLL cells were able to stimulate moderate T cell proliferation, but did not increase T cell mediated killing of CLL cells. However, CLL cells incubated with both S28690 and PDB (a PKC agonist), exhibited much lower amounts of phosphorylated STAT3, triggered marked T cell proliferation, and stimulated T cell mediated killing of CLL cells.

Conclusions: S28690 (a TLR7 agonist) causes increased expression of co-stimulatory molecules by CLL cells in vitro and transforms CLL cells into moderate stimulators of T cell proliferation. The effects of S28690 are synergistic with PKC agonists, potentially as a result of S28690-mediated NFkB activation and concurrent PKC-mediated inhibition of STAT3. These findings may find clinical application in immunotherapeutic approaches to CLL.

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