Bone marrow-derived macrophage lines and immortalized cloned macrophage and dendritic cells support priming of Borrelia burgdorferi--specific T cell responses in vitro and/or in vivo.

In vitro propagated bone marrow-derived macrophage populations (BMMO) as well as cloned immortalized macrophage (MT2/1) and dendritic (D2SC/1) cell lines were analyzed for their capacity to promote activation and/or proliferation of naïve T cells to Borrelia burgdorferi antigens in vitro and in vivo. All three cell types constitutively express high levels of MHC class I structures as well as the co-stimulatory molecules B7/BB1 and heat-stable antigen (HSA); MHC class II molecules (I-A) are upregulated following incubation with either intact spirochetes or the purified lipoprotein OspA (Lip-OspA) but not with its delipidated from (MDP-OspA). Only BMMO were able to induce proliferation of naïve T cells or T cells derived from infected mice to intact spirochetes in vitro. However, all three accessory populations could support primary and secondary T cell responses to Lip-OspA but not, or only marginally, to MDP-OspA under similar conditions. The number of accessory cells required for optimal stimulation of naïve or pre-sensitized T cells was approximately 3 x lower for D2SC 1 than for BMMO or MT2/1. In addition, BMMO pre-pulsed with Lip-OspA were able to prime T cells in vivo, indicating a crucial role for the lipid moiety in antigen presentation. From two truncated lipopeptides of Lip-OspA containing either 20 or 6 aminoterminal residues, only Lip-OspApep20 but not Lip-OspApep6 induced significant proliferation in naïve for pre-sensitized T cells in vitro, suggesting that T cells mainly respond to the protein rather than the lipid moiety of OspA. Thus, the data demonstrate that BMMO, MT2/1 and D2SC/1 have differential capacities to prime spirochete-reactive T cells and to support their growth in vitro, suggesting that optimal activation and propagation of T cells also depends on the quality of the antigen.