Human dendritic cells phagocytose and process Borrelia burgdorferi

There is strong evidence that the immune response to Borrelia burgdorferi (Bb) contributes to the pathogenesis of Lyme disease. Bb are transmitted by ticks to the skin, which is particularly rich in dendritic cells (DC). The initial reaction of these APCs may already set the course to immune pathogenesis. To study the role of DC, biopsies from human skin were incubated with Bb and investigated in toto with electron microscopy. In addition, DC freshly isolated from dermis (DDC) and epidermis (Langerhans cells) were compared with blood-derived DC (BDC). In situ, Bb were found in the dermal layer of the skin only, occasionally cleared by DDC. Isolated DDC and BDC, but not Langerhans cells, readily engulfed Bb preferentially using coiling phagocytosis. Internalized Bb were located free in the cytosol and inside of phagolysosomes of DDC and BDC. Intravesicular Bb Ags were colocalized with MHC class II molecules. In addition, live Bb induced IL-12 production in BDC. Bb-pulsed BDC activated naive and primed autologous Bb-specific T cells, as measured by detection of granulocyte-macrophage CSF gene transcription and proliferative response, respectively. These data indicate that human DC phagocytose, process, and present Bb Ags. The way in which DC may influence the immune response in Lyme disease, however, remains to be evaluated.