The aim of this study was to provide proof-of-concept for quantitative and qualitative label-free detection of
immune complexes through myeloid cells with imaging surface plasmon resonance. Surface plasmon resonance imaging was first applied to monitor the binding of human sera from healthy and
rheumatoid arthritis (RA) patients to immobilized citrullinated RA-specific
peptide antigens,
histone citrullinated
peptide 2 (HCP2) and
viral citrullinated peptide 2 (VCP2). Next, the binding of monocytoid cell line U937 to the resulting
immune complexes on the sensor surface was monitored. As control, binding of U937 was monitored to
immunoglobulin (Ig)G subclasses simultaneously. Cell response results were compared to results of
cyclic citrullinated peptide 2 (CCP2)
enzyme-linked
immunosorbent assay (ELISA), clinical RA diagnosis and
antigen-specific antibody distribution of the samples. Human
IgG3 triggered the most pronounced response, followed by
IgG1 and
IgG4, while
IgG2 did not result in U937 cell binding. Serum samples obtained from RA patients resulted in a significantly increased cell response to VCP2 compared to healthy controls. The strength of cell response towards VCP2
immune complexes showed significant correlation with levels of
antigen-specific
IgA,
IgG and
IgG3. Cellular responses on VCP2
immune complexes showed significant association with both CCP2-based serological positivity and European League Against
Rheumatism (EULAR) criteria-based clinical RA diagnosis.
Immunoglobulin-triggered binding of monocytoid cells can be monitored using a label-free multiplex technology. Because these binding events are presumably initiated by
Fc receptors, the system provides a tool for
biological detection of
autoantibodies with diagnostic value, here exemplified by anti-citrullinated
antibodies. This provides added information to antibody levels, as interaction with
Fc-receptor-expressing cells is also affected by post-translational modification of the
immunoglobulins.