Antigen uptake and presentation by ovine afferent lymph dendritic cells

Abstract

Dendritic cells (DC) are accessory cells distributed throughout non-lymphoid and lymphoid tissues, and also found in the blood and afferent lymph. These cells represent 1-10% of the leucocytes in ovine afferent lymph, and can be purified using pseudoafferent cannulation techniques. DC are essential as accessory cells for the initiation of primary immune responses, and are also the most effective antigen presenting cells for activation of T cells in the secondary response. However, the mechanisms underlying the potent accessory capability of DC are currently poorly understood. The aims of this thesis were to characterise DC surface markers and Fc receptor expression in primary and secondary immune responses, and to investigate the effect of enhanced antigen uptake via DC Fc receptors on T cell activation. The phenotype of DC from the afferent lymph of sheep was investigated by flow cytometry. MHC class I, class II and CD1 were observed at high levels on the DC surface, while staining for immunoglobulins was variable. Cells obtained from the afferent lymph of primed sheep on antigen challenge in vivo showed pronounced and rapid modulation of surface markers, which may be required for processing and presentation of antigen and migration to the draining lymph node.

Further characterisation of DC demonstrated staining with polyclonal antisera to peptides of the bovine Fc receptor Type I for IgG. Western blot analysis of DC lysates identified bands of the correct sizes for the Type I and II Fc receptors for IgG, suggesting that both are expressed by ovine afferent lymph DC.

Functional studies were carried out to investigate the role of Fc receptors for IgG in the uptake and presentation of antigen by DC. Specific antibody greatly increased the response of CD4+ T cells to substimulatory concentrations of antigen presented by DC, depending on the antigen/ antibody ratio. F(ab')2 portions of specific antibody and intact irrelevant antibody were completely ineffective. These results suggest enhanced presentation of antigen by DC following the uptake of immune complexes via Fc receptors. Similar results, although with a smaller degree of potentiation, were obtained using unfractionated peripheral blood mononuclear cells.

using unfractionated peripheral blood mononuclear cells. Fc receptors on DC would be important in secondary responses in vivo, in which specific antibody of IgG isotype is present. The potentiation of T cell activation may also play a role in diseases in which immune complexes are presented by DC.