The slan antigen identifies the prototypical non-classical CD16⁺-monocytes in human blood

Peripheral monocytes in humans are conventionally divided into classical (CL, CD14⁺⁺CD16^-), intermediate (INT, CD14⁺⁺CD16⁺) and non-classical (NC, CD14^dim/-CD16⁺⁺) cells, based on their expression levels of CD14 and CD16. A major fraction of the NC-monocytes has been shown to express the 6-sulfo LacNAc (slan) antigen, but whether these slan⁺/NC-monocytes represent the prototypical non-classical monocytes or whether they are simply a sub-fraction with identical features as the remainder of NC monocytes is still unclear.

We analyzed transcriptome (by bulk and single cell RNA-seq), proteome, cell surface markers and production of discrete cytokines by peripheral slan⁺/NC- and slan^-/NC-monocytes, in comparison to total NC-, CL- and INT- monocytes.

By bulk RNA-seq and proteomic analysis, we found that slan⁺/NC-monocytes express higher levels of genes and proteins specific of NC-monocytes than slan^-/NC-monocytes do. Unsupervised clustering of scRNA-seq data generated one cluster of NC- and one of INT-monocytes, where all slan⁺/NC-monocytes were allocated to the NC-monocyte cluster, while slan^-/NC-monocytes were found, in part (13.4%), within the INT-monocyte cluster. In addition, total NC- and slan^-/NC-monocytes, but not slan⁺/NC-monocytes, were found by both bulk RNA-seq and scRNA-seq to contain a small percentage of natural killer cells.

In addition to comparatively characterize total NC-, slan^-/NC- and slan⁺/NC-monocyte transcriptomes and proteomes, our data prove that slan⁺/NC-, but not slan^-/NC-, monocytes are more representative of prototypical NC-monocytes.