A numerical analysis model for interpretation of flow cytometric studies of ex vivo phagocytosis

Quantification of ZnO nanoparticle uptake, distribution, and dissolution within individual human macrophages

The usefulness of zinc oxide (ZnO) nanoparticles has led to their wide distribution in consumer products, despite only a limited understanding of how this nanomaterial behaves within biological systems. From a nanotoxicological viewpoint the interaction(s) of ZnO nanoparticles with cells of the immune system is of specific interest, as these nanostructures are readily phagocytosed. In this study, rapid scanning X-ray fluorescence microscopy was used to assay the number ZnO nanoparticles associated with ∼1000 individual THP-1 monocyte-derived human macrophages. These data showed that nanoparticle-treated cells endured a 400% elevation in total Zn levels, 13-fold greater than the increase observed when incubated in the presence of an equitoxic concentration of ZnCl2. Even after excluding the contribution of internalized nanoparticles, Zn levels in nanoparticle treated cells were raised ∼200% above basal levels. As dissolution of ZnO nanoparticles is critical to their cytotoxic response, we utilized a strategy combining ion beam milling, X-ray fluorescence and scanning electron microscopy to directly probe the distribution and composition of ZnO nanoparticles throughout the cellular interior. This study demonstrated that correlative photon and ion beam imaging techniques can provide both high-resolution and statistically powerful information on the biology of metal oxide nanoparticles at the single-cell level. Our approach promises ready application to broader studies of phenomena at the interface of nanotechnology and biology.

Increased uptake by splenic red pulp macrophages contributes to rapid platelet turnover in WASP(-) mice

Thrombocytopenia caused by rapid platelet consumption contributes to the severe thrombocytopenia of Wiskott-Aldrich syndrome (WAS) and to the milder thrombocytopenia seen in murine WAS. We show that rapid clearance of ¹¹¹In-labeled murine WASP(-) platelets correlates with enhanced splenic uptake. Using platelets labeled with a pH-sensitive fluorescent marker (pHrodo), we quantify normal platelet uptake by red pulp macrophages (RPMs), and demonstrate its enhancement after in vivo opsonization of platelets. The spleens of WASP(-) mice contain an increased number of RPM, and rapid clearance of WASP(-) platelets in WASP(-) mice in turn generates an increased number of pHrodo(+) splenic RPMs. To separately assess the platelet intrinsic and recipient-dependent functions involved in the clearance and splenic phagocyte uptake of WASP(-) platelets, we performed "crossed" pHrodo(+) platelet injection studies (wild type [WT] to WASP(-), WASP(-) to WT). We show that an extrinsic effect of recipient WASP deficiency on the clearance of WASP(-) platelets correlates with increased platelet uptake by RPMs. An intrinsic effect of platelet WASP deficiency on platelet clearance does not, however, correlate with increased total uptake by WT or WASP(-) RPMs. In contrast to other published findings, we find no evidence of a baseline or antibody-induced increase in phosphatidyl serine exposure on WASP(-) platelets. Our findings suggest that an increased number of RPMs in WASP(-) mice contributes significantly to the increased platelet consumption rate in WASP(-) mice. This might explain the consistent efficacy of splenectomy in murine and clinical WAS.

A numerical analysis model for the interpretation of in vivo platelet consumption data

Unlike anemias, most thrombocytopenias cannot be separated into those due to impaired production and those due to accelerated consumption. While rapid clearance of labeled platelets from the bloodstream can be followed in thrombocytopenic individuals, no model exists for quantitatively inferring from autologous or allogeneic platelet consumption data what changes in random consumption, lifespan dependent consumption, and platelet production rate may have caused the thrombocytopenia. Here we describe a numerical analysis model which resolves these issues. The model applies three parameter values (a random consumption rate constant, a lognormally-distributed platelet lifespan, and the standard deviation of the latter) to a matrix comprising a series of platelet cohorts which are sequentially produced and fractionally consumed in a series of time intervals. The cohort platelet counts achieved after equilibration of production and consumption both enumerate the population age distribution and sum to the population platelet count. Continued platelet consumption after production is halted then serves to model in vivo platelet consumption data, with consumption rate in the first such interval defining the equilibrium platelet production rate. We use a least squares fitting procedure to find parameter values which best fit observed platelet consumption data obtained in WT and thrombocytopenic WASP(-) mice. Equilibrium platelet age distributions are then ‘grafted’ into the matrix to allow modeling of the consumption of WT platelets in WASP(-) recipients, and vice versa. The optimal parameter values obtained indicate that random WT platelet consumption accounts for a larger fraction of platelet turnover than was previously suspected. Platelet WASP deficiency accelerates random consumption, and a trans effect of recipient WASP deficiency contributes to this. Application of the model to clinical data will allow distinctions to be made between thrombocytopenias due primarily to impaired platelet production and those due to acceleration of random or lifespan-dependent platelet consumption.

Efficient measurement of opsonising antibodies to Plasmodium falciparum merozoites

Background

Antibodies targeting merozoites are important in protection from malaria. Therefore, merozoite surface proteins are attractive vaccine candidates. There is a need for robust functional assays to investigate mechanisms of acquired immunity and vaccine efficacy. To date, the study of merozoite phagocytosis has been confounded by the complexity and variability of in vitro assays.

Methodology/Principal findings

We have developed a new flow cytometry-based merozoite phagocytosis assay. An optimized merozoite preparation technique produced high yields of merozoites separated from haemozoin. Phagocytosis by the undifferentiated THP-1 monocytic cell line was mediated only by Fc Receptors, and was therefore ideal for studying opsonising antibody responses. The assay showed robust phagocytosis with highly diluted immune sera and strong inter-assay correlation. The assay effectively measured differences in opsonisation-dependent phagocytosis among individuals.

Conclusions/Significance

This highly reproducible assay has potential applications in assessing the role of opsonic phagocytosis in naturally acquired immunity and vaccine trials.

Platelets from WAS patients show an increased susceptibility to ex vivo phagocytosis

The thrombocytopenia of Wiskott-Aldrich syndrome (WAS) is thought to be due to both reduced platelet production and accelerated platelet consumption. We have previously demonstrated that platelets from WASP-deficient mice are consumed more rapidly in vivo than are WT platelets, and that opsonization accelerates their uptake by bone marrow- derived macrophages more than it does that of WT platelets. Here we asked whether platelets from WAS patients show similar features. We show that ex vivo phagocytosis by activated THP-1 cells of DIO-labeled platelets from a series of WAS or XLT patients is increased in comparison to that of normal control platelets. Using a numerical analysis method, we distinguish this effect from a concurrent effect on the amount of detectable fluorescent signal transferred to the macrophage per phagocytosed platelet. We show that the latter quantity is reduced by platelet WASP deficiency, as might be expected if the fluorescence transferred from these smaller platelets is more rapidly quenched. We are unable to detect a differential effect of opsonization with anti-CD61 antibody on the uptake of WASP(-) vs. WT platelets. However, the high probability of phagocytosis per adsorbed WASP(-) platelet could limit the sensitivity of the assay in this case. We also see no effect of sera from WAS patients on the uptake of normal control platelets, suggesting that in vivo opsonization is not the cause of increased uptake of WASP(-) platelets. Finally, we show little, if any, increase in the reticulated platelet fraction in WAS patients, suggesting that impaired production of reticulated platelets contributes to the thrombocytopenia. Our findings suggest that rapid in vivo platelet consumption contributes significantly to the thrombocytopenia of WAS. They also demonstrate the feasibility of routinely performing functional assays of phagocytosis of small numbers of platelets obtained at remote locations, a method which should be applicable to the study of other types of thrombocytopenia such as ITP.