Background/Purpose: In RA, aberrant lymphocytes can damage synovial joints and other organs. Antigen-presenting cells (APC) can activate lymphocytes and are considered critical to initiate immune responses; we reason that APC may have relevance for the pathologic auto-immune response in RA. Surprisingly, we previously found a striking depletion of two potent APC lineages — CD141+ and CD1c+ myeloid dendritic cells — in RA. More interestingly, this was accompanied by the emergence of previously undetected cells (expressing unorthodox marker combinations, precluding their categorization into existing definitions) with APC potential (DR+). These cells expressed CD45RA, leading us to hypothesize that the inclusion of this isoform can help categorize myeloid APC more adequately.

Methods: RA patients satisfying the 2010 ACR classification criteria and matched healthy donors (n=10 each), underwent spectral cytometry to characterize myeloid APC in peripheral blood mononuclear cells. We gated myeloid APC based on forward and side scatter, HLA-DR expression and lack of markers for established monocytic lineages. We removed lymphocytes and non-viable cells using a CD3/CD19 dump and an amine-reactive viability dye. Plasmacytoid DC were excluded using CD303, resulting in a population containing established myeloid DC while retaining potentially new phenotypes; we refer to this definition as ‘candidate APC’ We then categorized candidate APC based on expression of CD45RA and CD1c. We calculated median fluorescence intensities of co-stimulatory/inhibitory molecules, and chemokine receptors. Kruskal-Wallis testing with a threshold of p< 0.05 was performed to assess for significant differences.

Results: Healthy donor APCs were dominated by CD1c+CD45RAint cells, consistent with established myeloid dendritic cells whereas in RA — as expected — myeloid dendritic cells were markedly decreased. (p = 0.019). Interestingly, we found that in RA two additional subsets, both characterized by much higher CD45RA expression, were significantly increased: CD45RAhi CD1c+ (p = 0.028) and CD45hiCD1c- (p = 0.049). In RA, the CD45RAhi subsets differed significantly in their expression of co-stimulatory/inhibitory and chemokine receptors: enhanced expression of CD86 in CD45RAhi CD1c- (p = 0.049). Both CD45RAhi populations showed dramatically higher expression of CD197 (CCR7; p < 0.01) compared with CD45RAint myeloid DC while exhibiting a lower expression of CD155 (poliovirus receptor; p < 0.01). CD83 and CD275 (ICOS-L) expression did not differ.

Conclusion: The blood of RA patients contains HLA-DR+ cells that do not fit existing definitions. These potential APC can be effectively captured by gating all DR+ cells which, with the addition of CD45RA and CD1c, can be categorized into subsets that suggest migratory and co-stimulatory capacity distinct from established CD1c+CD45RAint DC. A higher expression of CCR7, and in the case of the CD1c- subset, CD86, shows the potential for these putative APC to migrate to lymph nodes and, via CD28, provide pro-inflammatory stimulation to T lymphocytes.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/expansion-of-hla-drcd45rahi-non-lymphoid-cells-in-patients-with-rheumatoid-arthritis/