Background: Maturation of dendritic cells (DCs) by a pulse TLR ligands increases their antigen presentation efficacy and leads to increased proliferation of T cells in allogeneic mixed leukocyte cultures. Paradoxically, chronic TLR stimulation of DCs leads to an inhibition of T cell proliferation. The mechanisms and purpose of this phenomenon are not well understood.

Methods: BALB/c bone marrow-derived DCs were produced by culture with GM-CSF. Purified CD8+ and CD4+ T cells in the B6 background with direct alloreactivity toward BALB/c antigen were purified from 2C and 4C T cell receptor transgenic mice, respectively. T cells were assayed for proliferation by FACS by CFSE dilution and Ki-67 staining in response to unstimulated, TLR ligand pulsed, and chronically TLR stimulated DCs. Alternatively, T cells were stimulated with CD3+CD28+ beads with our without TLR lignads. TLRs tested were Pam3CSK4, Poly(I:C), LPS, heparan sulfate, and CpG which activate TLR2, 3, 4, and 9. Specific small molecule inhibitors of intracellular kinases pathways were tested for their ability to prevent proliferation block of alloreactive T cells in response to DCs chronically exposed to these ligands.

Results: Co-culture of DCs with TLR agonists resulted in the inhibition of proliferation of alloreactive murine T cells. This inhibition required DCs in culture and did not occur when stimulated by CD3 and CD28 alone. Amongst intracellular kinase inhibitors tested, the Janus kinase 3 (JAK3) inhibitor, CP-690550, resulted in a release of inhibition of T cell proliferation accompanied by the inhibition of CD25 upregulation. Downstream of activating cytokine receptors, the observed inhibition may be the result of the induction of suppressor of cytokine signaling (SOCS) proteins.

Conclusions: Pathogen-derived and endogenously-derived TLR ligands are critical regulators of adaptive immune responses. Differences in the abilities of DCs to stimulate T cell proliferation caused by transient exposure compared with chronic exposure has implication as to how TLR-activated DCs affect T cell responses at sites of active inflammation compared with distal sites of T cell priming.

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