Deficient efferocytosis (i.e. phagocytic clearance of apoptotic cells) has been frequently reported in systemic lupus erythematosus (SLE). Todate, patients with primary Sjögren's syndrome (SS) have not been assessed for phagocytosis of apoptotic cells (ApoCell-phagocytosis) and of particulate targets (microbeads, MB-phagocytosis).
ApoCell-phagocytosis and MB-phagocytosis were comparatively assessed by flow cytometry in peripheral blood specimens and monocyte-derived macrophage (MDM) preparations from healthy blood donors (HBD) and consecutive SS, SLE and rheumatoid arthritis (RA) patients. Cross-admixture ApoCell-phagocytosis experiments were also performed using phagocytes from HBD or patients, and apoptotic cells pretreated with whole sera or purified serum IgG derived from patients or HBD.
Compared to HBD, approximately half of SS and SLE patients studied (but not RA) manifested significantly reduced ApoCell-phagocytosis (p<0.001) and MB-phagocytosis (p<0.003) by blood-borne phagocytes that correlated inversely with disease activity (p≤0.004). In cross-admixture assays, healthy monocytes showed significantly reduced ApoCell-phagocytosis when fed with apoptotic cells that were pretreated with sera or purified serum IgG preparations from SS and SLE patients (p<0.0001, compared to those from HBD or RA). Such aberrant effect of the SS and SLE sera and IgG preparations correlated linearly with their content of IgG antibodies against apoptotic cells (p≤0.0001). Phagocytic dysfunction maybe also present in certain SS and SLE patients, as supported by deficient capacity of MDM for ApoCell-phagocytosis and MB-phagocytosis under patients' serum-free conditions.
Similarly to SLE, efferocytosis is frequently impaired in SS and is primarily due to the presence of inhibitory IgG anti-ApoCell antibodies and secondarily to phagocytes' dysfunction.