Identification and characterisation of novel human dendritic cell progenitors

Dendritic cells (DCs) are professional antigen presenting cells that initiate the immune response. They are heterogeneous and are broadly classified into three groups – two major subsets of conventional DCs (cDC) named cDC1 and cDC2, and plasmacytoid DC (pDC). Each subset has unique transcription factor dependency, and specialised functions. DCs are derived from a unique lineage of DC-restricted progenitors that are until now not yet fully characterised. Currently, there are limited genetic lineage fate mapping mice models available to track the development of DCs in steady state and inflammation. We have identified Uroplakin-1b (Upk1b) as uniquely expressed in DC-restricted progenitors. Surprisingly, this molecule is also expressed in microglia. We developed a fate mapping Upk1b-cre mouse, which was crossed to Rosa-YFP reporter mouse to generate Upk1b-cre:Rosa YFP. Contrary to our expectations, we observed low recombination in DC-restricted progenitors and DC subsets. However, the recombination level in the microglia was close to 80% in young adults, suggesting that it may be a useful model for microglial research. Upk1b is conserved across species and may serve as a potential marker to identify DC-restricted progenitors in humans. Although human DC-restricted progenitors were recently identified, here, we extended and further refined the definition of the precursor of cDC (pre-cDC). We showed that they share several common phenotypic markers such as CD303, CD123 and CD45RA with pDC, thereby contaminating the pDC fraction of the peripheral blood. Hence, this might explain why pDC when stimulated with IL-3 and CD40 ligand, were found to differentiate into cells with cDC-like morphology. We also demonstrated that pre-cDC but not pDC were able to differentiate into cDC subsets in MS5 stromal culture supplemented with the cytokines Flt3L, SCF and GM-CSF. We further interrogated the bulk pre-cDC population in peripheral blood and identified three populations of pre-cDC, namely early, uncommitted pre-cDC and committed pre-cDC1 and pre-cDC2. Early, uncommitted pre-cDC differentiated into both cDC subsets, while committed pre-cDC1 and pre-cDC2 differentiated into their respective cDC subsets. We also observed that they are responsive to Flt3L stimulation and were able to induce allogeneic naïve T cell proliferation in the absence of stimulation. Finally, we observed that pre-cDC was expanded in the blood of Systemic Lupus Erythematosus (SLE) patients, and are currently investigating the role of pre-cDC in this autoimmune disease.DOCTOR OF PHILOSOPHY (SBS