CD16+ Dendritic Cells Are a Unique Myeloid Antigen Presenting Cell Population

Dendritic cells (DC) are phenotypically identified in human blood as HLA-DR⁺ cells, which lack major cell surface lineage markers. We demonstrated that myeloid antigen presenting cells, including monocytes and DC display a continuum of CD14 and CD16 expression (10^th Human Leucocyte Differentiation Antigen Workshop). The robustness of DC and monocyte identification, particularly when identifying cell subsets with little or no surface CD14 is limited by subjective gating strategies for determining rare cell populations. Application of Poisson counting statistics established that rare cell types such as "CD14^- CD16⁺ DC" are often overlooked in analyses powered to detect the much larger populations of "classical" and "non-classical" monocytes.

We used fluorescent and mass cytometry, in conjunction with unsupervised high dimensional clustering, to show that the continuum of CD14 expression separates CD14^lo CD16⁺ non-classical monocytes and CD14^- CD16⁺ DC. We have defined the CD14^-CD16⁺ DC using a broad panel of cell surface markers and established a CD14^-CD16⁺ DC phenotypic signature that is distinct from both classical and non-classical monocytes in healthy donor blood. The CD14^-CD16⁺ DC differ in both size to CD14^lo CD16⁺ monocytes and functional antigen uptake, with slower kinetics of soluble antigen uptake into lysozymes. Their proteasome processing and presentation of influenza matrix protein by MHC I was comparable to other primary blood monocytes and DC antigen presenting cell populations. The CD14^-CD16⁺ DC had limited capacity for further in vitro differentiation. The recovery of CD14^-CD16⁺ DC after autologous and allogeneic myeloablative hematopoietic cell transplants (HCT) followed similar kinetics to other monocytic and DC populations. CD14^lo CD16⁺ monocytes expressed CCR5 as did other myeloid DC but CD14^-CD16⁺ DC lacked CCR5, although interferon induced CCR5. The early differentiation and induction of CCR5 on circulating CD16⁺ DC after allogeneic HCT predicted for the onset of acute graft versus host disease.

These data demonstrate that "CD14^- CD16⁺ DC" represents a distinct clinically relevant human white blood cell population, whose ontogeny and function are under further investigation.