Possible reasons for variable leukapheresis collection outcomes with automated apheresis systems

A new cell separator with automated interface management system (Spectra Optia, Terumo BCT, Lakewood, CO) is currently compared to longer-established apheresis systems aiming to elucidate the best program settings for the efficient mononuclear cell (MNC) apheresis in non– cytokine-stimulated donors. This is challenged by the observation of noticeably variable collection efficiencies (CEs) between different donors and MNC populations. Schulz and coworkers recently described the fact that CE variability can be only partly explained by interindividual differences in donors and assumed a possibly large operation-dependent variability of the Spectra Optia device. So far, only small numbers of nonmobilized healthy donors (not exceeding 20 donors in a cohort) and highly different processed blood volumes were investigated. While the latter accounts for a limited comparability of target cell yields, a wide range of CE1 values between these analyses was obtained. In particular, CE1 for T cells spread between 20% and more than 85%, which is likely relevant for donor lymphocyte infusion (DLI) collections. Recently, we found a dependency of CE1 for T cells but not for monocytes and CD34+ cells on the number of chamber flushes using the Spectra Optia device. MNC apheresis with at least three chamber flushes resulted in significantly higher CE1 values for T cells compared to apheresis procedures with only two chamber flushes (mean CE1, 52.1% vs. 26.2%). Accordingly, a different frequency of chamber flushes is likely to explain different lymphocyte collection results when comparing our data to DLI collections by Schulz and coworkers (mean CE1, 44% [T cells] vs. 64% [lymphocytes]). In contrast, CE1 for monocytes, which is more insensitive relative to cycle volume, was in our apheresis procedures higher compared to the data of Schulz and coworkers (mean CE1, 67.1% vs. 53.6%). These data, revealing diverging CEs within the MNC population, tend to support the hypothesis that rather late chamber flushing can result in a selective loss of possibly smaller diameter lymphocytes within the MNC population by elutriation from the top of the chamber or entrainment with other smaller particles such as concentrated platelets (PLTs) and red blood cells (RBCs) in the late cycle stage. Thus, in DLIs seeking for high T cell yields, target cell loss might be avoided by a volume-based trigger early in the procedure, albeit with the disadvantage of an increase in the PLT CE and subsequent higher PLT loss in donors. Chamber emptying can be also automatically performed by multistep algorithms and the advanced optical detection systems. Therefore the RBC trigger can be either initiated by a sufficient change in red to green absorbance or a changed signal indicating an alteration in cellularity of the stream or a peak RBC concentration exceeding a preset value. Notably, by comparing the cell concentrations of the MNC products between mobilized and nonmobilized apheresis procedures, Schulz and coworkers hypothesized in nonmobilized collections a premature RBC trigger before the chamber reached capacity. However, in our analyses we found rather delayed chamber flushings with respect to cycle volumes and CE of T cells. Clarifying these phenomena requires an analysis of the production logs (DLOGs) to provide detailed information for identifying the course of the RBC triggering algorithm. In conclusion, we assume that the timing of chamber flushing is essential for increasing CEs of T cells. Variability of collection results will, however, persist, which is not only subjected to the operational apheresis procedure but to individual differences among donors, as it has been already shown for manual cell separators. This phenomenon apparently still exists even after introducing highly automated apheresis systems.