Dynamic expression of complement receptor immunoglobulin (CRIg) on monocytes and its role in phagocytosis and killing of Staphylococcus aureus

Background: The complement receptor immunoglobulin (CRIg), a key microbial pathogen phagocytosis-promoting receptor, responsible for intravascular clearance of bacteria, is purported to be expressed selectively on tissue-fixed macrophages such as Kupffer cells. However, recently it has been reported that neutrophils can also express functional CRIg following activation by inflammatory mediators. Monocytes have been reported not to express CRIg under non-activated conditions. Thus, investigations were undertaken to examine whether blood monocytes express CRIg under cell activation conditions and its role in anti-microbial immunity. 

Methods: Monocytes CRIg expression in whole human and mouse blood or peripheral blood mononuclear cells and purified monocytes using density gradient centrifugation or an affinity purification kit was examined using PE/FITC-labelled anti-CRIg monoclonal antibody and flow cytometry. Characterization of CRIg isoforms in monocytes was determined by the detection of CRIg mRNA transcripts and protein using RT-PCR and Western blot, respectively. Gene-edited CRIg^– and CD18^– monocytic THP-1 cell lines were generated to assess the role of CRIg and CD18 in cell adhesion, phagocytosis, and microbial killing. Functional assays were performed using Staphylococcus aureus as a model pathogen. 

Results: CRIg was constitutively expressed, dynamically, on the surface of human and mouse blood monocytes. All three human monocyte subpopulations expressed CRIg, equally. The inability to demonstrate expression on monocytes cell surface by previous studies can be explained by its lability during blood storage and loss during monocyte isolation steps. Interestingly of the monocyte subpopulations only the classical and intermediate but not the non-classical showed a loss of CRIg expression. The data showed that loss from the surface was most likely due to relocation of the receptor intracellularly. Monocytes expressed 6 different CRIg mRNA transcripts and immunoreactive isoforms. Using CRIg^– and CD18^– THP-1 monocytic cells, we found that both CRIg and CD18 (CR3/CR4) were critical for cell adhesion, but for phagocytosis and killing of S. aureus, either receptor was independently effective. 

Conclusion: The data provide compelling evidence that monocytes express functional CRIg, relevant to the cells’ anti-microbial role of the ‘wandering’ phagocyte and consolidate a view that CRIg is widely expressed in our phagocytic cell system, similar to the classical complement receptors CR3 and CR4.