Functional Studies on the C-Type Lectin Receptor CD302 Present on Dendritic Cells and Macrophages

Introduction: C-type lectin receptors (CLR) play an important role in the immune system by recognising molecular patterns expressed by exogenous and endogenous threats. They have been shown to capture and internalise antigens and to mediate other important immune cell functions. DEC205 and CLEC9A are being actively investigated as targets for clinical therapeutic cancer vaccines. We discovered CD302 as a new CLR expressed on human dendritic cells (DC), monocytes and macrophages (J Immunol 2007;179:6052). Our initial studies suggested the molecule could play a role in cell adhesion or migration due to its co-localisation with migratory structures on macrophages. Our study set out to investigate the potential immunological function of CD302 using mouse models and to define its wider tissue expression in man.

Methods: We generated CD302 knockout (KO) mice lacking exon 1 of its gene, abrogating transcription, for functional studies. We characterised the transcriptional expression of CD302 in mouse immune cells using real-time PCR. We developed monoclonal mAb to mCD302. Human studies utilized the anti-CD302 mAbs, MMRI-20 & 21 in flow cytometry and confocal microscopy studies of human immune cell populations.

Results: CD302 was primarily expressed in mouse liver, lungs, lymph nodes (LN) and spleen. In spleen, macrophages, granulocytes and dendritic cells (DC) expressed CD302. Analysis of LN DC subsets revealed 2.5-fold higher CD302 mRNA expression in migratory compared to resident DC populations. Enumeration of various immune populations in lymphoid organs by flow cytometry uncovered a modest deficiency in migratory DC number and proportion within LN of CD302 KO mice compared to wild-type (WT) mice. In vitro studies showed CD302 KO and WT DC had an equivalent capacity to be activated by various stimuli, prime T cells and migrate towards the lymphoid-homing chemokines CCL19/CCL21. CD302 KO migratory DC exhibited a reduced in vivo migratory capacity to LN after FITC skin-painting. However, CD302 KO macrophages migrated similarly to WT macrophages in vivo in response to thioglycollate. In man, CD302 was present in high density in liver and peripheral blood monocytes and myeloid but not plasmacytoid DC. Current studies are aimed at clarifying its distribution on tissue DC and macrophage subsets. Anti-CD302 coated microbeads were taken up by human monocyte derived macrophages and anti-CD302 mAb was also internalized by DC. Confocal studies showed that CD302 co-localized with F-actin structures at the near basal surface such as filopodia and lamellipodia and podosomes of human macrophages and EGFP tagged CD302 expressed in COS-1 cells associated with F-actin.

Conclusion: Our data suggests that CD302 may play a specialist role in DC and macrophage membrane functions. This appears to relate to its ability to associate with F-actin and may contribute to the membrane interactions required for DC to migrate towards the draining LN.