Previous studies have identified a 210 000-molecular weight molecule expressed at a high level on the surface of dendritic cells (DCs) in afferent lymph of cattle and evident on cells with the morphology of DCs in lymphoid tissues. Expression is either absent from other immune cells or is present at a lower level. The molecular weight and cellular distribution suggested that the molecule, called bovine WC6 antigen (workshop cluster), might be an orthologue of human DEC-205 (CD205). To establish whether this was the case, the open reading frame of bovine DEC-205 was amplified, by polymerase chain reaction, from thymic cDNA (accession no. AY264845). The cDNA sequence of bovine DEC-205 had 86% and 78% nucleic acid identity with human and mouse molecules, respectively. COS-7 cells transfected with a plasmid containing the cattle DEC-205 coding region expressed a molecule that stained with WC6-specific monoclonal antibody, showing that ruminant WC6 is an orthologue of DEC-205. Two-colour flow cytometry of mononuclear cells from afferent lymph draining cattle skin, and from blood, confirmed the high level of expression on large cells in lymph that were uniformly DC-LAMP positive and major histocompatibility complex class II positive. Within this DEC-205(+) DC-LAMP(+) population were subpopulations of cells that expressed the mannose receptor or SIRPalpha. The observations imply that DCs in afferent lymph are all DEC-205(high), but not a uniform population of homogeneous mature DCs.

As a consequence of the central role of dendritic cells (DC) in stimulating primary immune responses any bias in the response introduced by the DC has the potential for having a long-term effect on immunity. Examination and analysis of ruminant afferent lymph dendritic cells derived by cannulation allows studies on the properties of ex vivo DC that is not possible in humans and rodents and information can be derived from ruminants that has implications of generic relevance. Previous studies have identified two major populations of DC in afferent lymph draining the skin of cattle that differ in their capacity to stimulate CD4 and CD8 T cells. Differences in expression of cytokine transcripts have now been shown for the two types of DC. The CD11a(+)/SIRPalpha(-) population synthesised more IL-12, whilst the CD11a(-)/SIRPalpha(+) population produced more IL-10. This is likely to affect the bias of the immune response following presentation of antigen to T cells by one DC sub-population or the other. An inability to synthesise IL-1alpha was the reason for the failure of the CD11a(+)/SIRPalpha(-) DC to stimulate CD8 T cells. This property would potentially affect the induction of CD8 responses. Expression of the co-stimulatory molecules CD80, CD86 and CD40 appeared similar for both DC populations and not to relate to differences in function. A further examination of the SIRPalpha molecule on DC indicated that on cross-linking it was tyrosine phosphorylated and that it recruited the SHP-2 protein phosphatase. Associated with this was a blocking of TNFalpha secretion on exposure to LPS. The interaction of SIRPalpha with its ligand CD47 on T cells appeared to be an early event in the stimulation of T cells as binding of the ligand was reduced on activated T cells. CD26 was identified as another molecule expressed by the SIRPalpha(-) DC sub-population. This is reported to have an enzymatic activity on certain chemokines that could result in the promotion of a Th1 bias.A model is proposed that takes these observations into account in which SIRPalpha(-) DC would be expected to promote a Th1 biased response and the SIRPalpha(+) DC a more balanced one.

Two major sub-populations of dendritic cells (DC) are present in afferent lymph draining the skin of cattle distinguished by expression of signal regulator protein alpha (SIRPalpha). The SIRPalpha(-) population expresses the uncharacterized bovine WC10 antigen (Ag). Initial N-terminal sequencing of the WC10 protein purified by affinity chromatography showed significant homology with human CD26. A cDNA encoding bovine CD26 was cloned and the recombinant molecule expressed in COS-7 cells. Transfectants abrogated the ability of macrophage-derived chemokine (MDC) to cause a calcium flux in bovine PBMC indicating enzymatic activity characteristic of CD26. They also stained with WC10 monoclonal antibody confirming that the Ag is CD26. This is the first description of CD26 expression by DC in vivo or in vitro. It is expressed on a sub-population of ex vivo DC in afferent lymph draining the skin and on sub-populations of DC isolated from prescapular and mesenteric lymph nodes draining the skin or intestine, respectively. CD26 is an exopeptidase with specificity for motifs within the receptor-binding domain of several chemokines including MDC. CD26 mediated truncation of MDC affects the Th cell response effected by the chemokine and may produce a Th1 bias. Transcripts for MDC were present in both CD26(+) and CD26(-) DC, thus CD26 mediated modification of MDC may bias the immune response induced in naive T cells by DC.