Uremia-related oxidative stress in leukocytes is not triggered by β2-microglobulin.
BACKGROUND
Chronic kidney disease (CKD) is characterized by low-grade inflammation and increased risk for cardiovascular disease. The interest in β2-microglobulin (B2M) as a marker for cardiovascular outcome with and without CKD has grown. Clinical studies suggested that B2M could be involved in the pathogenesis of vascular disease, for which chronic leukocyte activation is a pathogenic factor. We investigated whether B2M is proinflammatory by inducing oxidative burst in leukocytes.


METHODS
Oxidative burst was measured at baseline and after stimulation with N-formyl-methionine-leucine-phenylalanine (fMLP), Escherichia coli, or phorbol-12-myristate-acetate (PMA) in the whole blood of healthy volunteers in the absence (saline) and presence of human B2M (hB2M; 10 and 50 mg/L) versus uremic whole blood. Because of suspicion of contamination, hB2M was dialyzed for purification and purified B2M (dB2M) and dialysates were tested in the burst test. As a comparator, reactive oxygen species (ROS) in response to lipopolysaccharide (LPS) was measured.


RESULTS
Unpurified hB2M strongly enhanced ROS in monocytes and granulocytes after E. coli and PMA and moderately after fMLP stimulation compared with control (P < .01) and uremia (P < .01) whereas at baseline hB2M only induced ROS in granulocytes (P < .05). After purification, dB2M no longer increased burst activity, suggesting that contamination was responsible for the initial effect. An endotoxin concentration of less than 1.5 EU/mL, as observed in hB2M, could not induce oxidative stress.


CONCLUSION
This study suggests that B2M, a traditional marker for middle molecule retention and a novel marker for cardiovascular outcome, may not by itself cause vascular damage by influencing inflammatory response due to induction of leukocyte free radical production. However, an effect on other cell types involved cannot be excluded. Our data further reveal that this type of research might be skewed by non-LPS contaminants, and that care should be taken to exclude this bias.