The Mean Filtration Pressure of Leukocyte Suspensions and Its Relation to the Passage of Leukocytes through Nuclepore Filters and Capillary Networks

Objective: To quantitatively evaluate the deformability of bulk suspensions of leukocytes (WBCs), account for the variance of individual cell mechanical properties, and deduce the potential for WBC entrapment within the capillary network in response to alterations in cell properties. Methods: The transient washout of WBCs initially trapped under low perfusion pressure in 5-μm pores of Nuclepore filters was analyzed for a 0.2-ml bolus of WBCs (derived from hamsters) with equal numbers of filter pores and cells, to characterize the statistical distribution of pressures, p_yield, required to dislodge the cells. Contributions of the variance in WBC diameter, pore diameter, and cremaster muscle capillary diameter to p_yield were estimated with a WBC cortical shell model and an analysis of the probability function of the ratio of WBC to pore diameter, Λ. Results: For normal WBCs, p_yield exhibited a log-normal distribution with mean ((p_yield)) of 0.59 cmH₂O. Incubation of cells in cytochalasin-B reduced (p_yield) almost 50%, whereas phorbol myristate acetate increased (p_yield) twofold. Incubation in N-formyl-methiolnyl-leucyl-phenylalanine had no significant effect on (p_yield), as polymorphonuclear cells became permanently trapped in the filter. The fluorescent dyes acridine orange, acridine red, and tetramethylrhodamine isothiocyanate increased (p_yield) as much as 10-fold, whereas steady-flow filtration methods showed no alteration. Analysis of the distribution of Λ revealed that due to their smaller pore diameters, in vitro filtration methods may overestimate in vivo values of (p_yield) by almost twofold. Conclusions: The transient filtration of WBC suspensions appears to be much more sensitive to subtle alterations in WBC deformability than steady-flow methods and may provide greater insight into the determinants of capillary perfusion. Estimates of (p_yield) are comparable to those obtained with micropipettes and permit analysis of substantially greater numbers of cells within a sample. Fluorescence labeling techniques should be used with caution, as they may dramatically alter cell properties to an extent undetectable by direct in vivo observations.