DIALYSIS NEUTROPENIA - THE ROLE OF THE CYTOSKELETON

The role of the cytoskeleton. Dialysis neutropenia is the result of pu lmonary sequestration of neutrophils after complement activation by th e dialyzer membrane. Increased expression of neutrophil adhesion recep tors, such as CD11b/CD18, suggests that neutrophil adhesion to the cap illary endothelium is a possible mechanism. An alternative hypothesis is that the complement fragment C5a modulates neutrophil mechanical pr operties via the cytoskeleton-largely filamentous actin (F-actin)-stif fening them and thereby slowing their passage through the pulmonary ca pillaries. To investigate this hypothesis, we developed an assay to me asure the F-actin content of neutrophils in whole blood using flow cyt ometry and the stain NBD-phallacidin. We measured neutrophil F-actin c ontent during hemodialysis of patients with polysulfone (N = 6), Hemop han (N = 6), and Cuprophan membranes sterilized with either ethylene o xide (N = 5) or steam (N = 6). Cell counts, neutrophil and monocyte CD 11b expression and plasma C5a concentrations were also measured. The r esults confirm the strong relationship between the degree of neutropen ia, increases in CD11b expression and plasma C5a levels reported by pr evious researchers. Modulation of the F-actin content of neutrophils w as also strongly related to C5a levels, indicating that the neutrophil cytoskeleton is active during dialysis. Modeling of cell counts sugge sts that with Cuprophan a substantial fraction of neutrophils and mono cytes are sequestered before they even pass through the dialyzer, sugg esting some form of systemic activation of these cells. Evidence for s ystemic activation was also seen in measurements of F-actin content, b ut not CD11b expression, a finding that strengthens the case for the i nvolvement of the cytoskeleton in dialysis neutropenia.