Standardization of lymphocyte antibody binding capacity - a multi-centre study

Development of an 18F-labeled anti-human CD8 VHH for same-day immunoPET imaging

PURPOSE

Cancer immunotherapies (CITs) have revolutionized the treatment of certain cancers, but many patients fail to respond or relapse from current therapies, prompting the need for new CIT agents. CD8⁺ T cells play a central role in the activity of many CITs, and thus, the rapid imaging of CD8⁺ cells could provide a critical biomarker for new CIT agents. However, existing ⁸⁹Zr-labeled CD8 PET imaging reagents exhibit a long circulatory half-life and high radiation burden that limit potential applications such as same-day and longitudinal imaging.

METHODS

To this end, we discovered and developed a 13-kDa single-domain antibody (VHH5v2) against human CD8 to enable high-quality, same-day imaging with a reduced radiation burden. To enable sensitive and rapid imaging, we employed a site-specific conjugation strategy to introduce an ¹⁸F radiolabel to the VHH.

RESULTS

The anti-CD8 VHH, VHH5v2, demonstrated binding to a membrane distal epitope of human CD8 with a binding affinity (K_D) of 500 pM. Subsequent imaging experiments in several xenografts that express varying levels of CD8 demonstrated rapid tumor uptake and fast clearance from the blood. High-quality images were obtained within 1 h post-injection and could quantitatively differentiate the tumor models based on CD8 expression level.

CONCLUSION

Our work reveals the potential of this anti-human CD8 VHH [¹⁸F]F-VHH5v2 to enable rapid and specific imaging of CD8⁺ cells in the clinic.

Flow cytometric profiles, biomolecular and morphological aspects of transfixed leukocytes and red cells

BACKGROUND

We evaluated the effects of the TransFix(TM) short-term stabilization technique on leukocyte subpopulations in both optimal and adverse storage temperatures and on different cellular concentrations. Particularly, we analyzed DNA cell content and membrane structure also for erythrocytes using a multiparametric approach.

METHODS

We studied biomolecular and morphological aspects of transfixed cells, by means of SEM, TEM, Western blotting, and by flow cytometry (FC). Furthermore, FC, Tunel, and electrophoresis were applied to evaluate DNA behavior.

RESULTS

We confirm preservation of scatter characteristics and immunophenotyping, extending such evaluations to cells stored in suboptimal conditions (25 degrees C and 37 degrees C) and in high density. Data demonstrate for lymphomonocytic cells an optimal conservation, slightly decreasing at higher temperatures for both 1/5 and 1/10 ratio (TransFix/sample), with enhanced autofluorescence. Eosinophils, basophils, and neutrophils are shown to preserve differently over time. The three different cellular concentrations evaluated (30,000-120,000 cell/microl) demonstrate substantial stability in FI values. Furthermore DNA content analysis attests the absence of any apoptotic pattern. Transfixed red cell protein profile as well as their morphological features appears almost unaltered.

CONCLUSIONS

Cytometric performance is suboptimal in aged unfixed specimens because of apoptosis that affects light scatter properties. Our findings highlight that lymphomonocytic cells are well stabilized even at suboptimal temperature and cell density. TransFix is able to abolish any apoptotic features and acts as an optimal blood preservative for appropriate preanalytical stabilization.

Risk factor correlates of platelet and leukocyte markers assessed by flow cytometry in a population-based sample

BACKGROUND

Platelet and leukocyte products are involved in atherothrombosis. However, the determinants of platelet and leukocyte markers assessed by flow cytometry have not been documented in a population-based sample.

METHODS AND RESULTS

We performed flow cytometry on blood from participants (n=1894) in the Atherosclerosis Risk in Communities (ARIC) Carotid MRI Study. Cellular aggregates and multiple platelet and leukocyte markers, such as myeloperoxidase in granulocytes and toll-like receptor-4, CD14, and CD45 in monocytes, were quantified. Their cross-sectional associations with demographic and risk factors were assessed using multiple linear regression. Mean values of most cellular markers and aggregates were considerably higher in blacks than whites (p<0.01). There were some differences in cellular markers between men and women, but little association with age. LDL-cholesterol was associated positively with several markers (toll-like receptor-4 and myeloperoxidase in granulocytes and CD162 in lymphocytes). Cholesterol-lowering therapy tended to show opposite associations. Smokers had much higher granulocyte myeloperoxidase than nonsmokers. However, most other correlations between risk factors and cellular markers were nonsignificant.

CONCLUSIONS

Race/ethnicity, sex, and to a lesser degree LDL-cholesterol and cholesterol-lowering therapy, but few other risk factors, were correlated with markers of cellular activation in this population-based study.

Atherosclerosis Risk in Communities (ARIC) Carotid MRI flow cytometry study of monocyte and platelet markers: intraindividual variability and reliability

BACKGROUND

Cellular markers help identify different components of a pathological process and may contribute to the diagnosis, prognostic assessment, and management of patients with suspected syndromes. Flow cytometry can be used to accurately assess markers of platelet and leukocyte activation and cellular aggregation in whole blood. To use cell markers as predictors of disease requires that they be measured reliably and show modest within-individual, day-to-day variation.

METHODS

We used whole blood flow cytometry to analyze monocyte and platelet markers in the Atherosclerosis Risk in Communities (ARIC) Carotid MRI study. We estimated laboratory variability using 20 split samples, process variation using replicate blood tubes taken from 112 subjects, and biologic plus process variation using replicate blood samples taken 4-8 weeks apart from 55 people.

RESULTS

For most analytes, the laboratory CV was <10% (mean 3.6%, range 0%-14.5%) and reliability was excellent (75% of analytes had R > 0.90). Reliability coefficients based on repeat-visit data indicated substantial to high repeatability (R > 0.60) for CD14, Toll-like receptor (TLR)-2, CD162, CD61, CD41, CD62P, CD154, and platelet-leukocyte aggregates. In contrast, TLR-4, CD45, myeloperoxidase (MPO), and cyclooxygenase (COX)-2 had slight to moderate repeat visit reliability.

CONCLUSIONS

The high repeatability results for selected platelet and monocyte markers indicate that they can be reliably measured in multicenter studies with delayed sample processing, provided that rigorous standardization of sample collection, shipping, and flow cytometry procedures is applied.

Evaluation of leukocyte stabilisation in TransFix®-treated blood samples by flow cytometry and transmission electron microscopy

In this report, we have evaluated the effects of a TransFix-based stabilisation technique on leukocyte scatter characteristics, immunophenotyping, membrane permeability, absolute cell counting and morphology to extend previously reported flow cytometric data focused on the lymphocyte population. We show that scatter characteristics, immunophenotyping and absolute cell counting are well preserved, particularly in the lymphocyte population. Nevertheless, a general increase in membrane permeability, evaluated by propidium iodide (PI) uptake, was observed in TransFix-treated leukocyte subsets. Ultrastructural observations show selective morphological preservation (up to 10 days of storage) of lymphocytes and, to a lesser extent, of monocytes. In contrast, granulocytes have necrosis-like features, although the plasma membrane seems well preserved. Therefore, electron microscopy observations reflect modifications induced in different cell populations as evidenced by flow cytometry (FC). The data indicate that this short-term stabilisation method is particularly suitable for the analysis of human lymphocytes and it is a good procedure for quality control programmes for inter- and intra-laboratory performance evaluation; good results are obtained with respect to antigen definition and absolute cell counting procedures. Any apoptotic pathways in leukocyte subsets are blocked for at least 10 days.

A standardized procedure for quantitation of CD11b on polymorphonuclear neutrophil by flow cytometry: potential application in infectious diseases

An up-regulation of the surface marker CD11b has been demonstrated during polymorphonuclear (PMN) cell activation. CD11b over-expression is often associated with inflammation and is considered as an early marker of infection. However, the absence of standardized assay and the variability of preanalytical settings leading to PMN artifactual activation have compromised the interest of this marker. In the present study a standardized quantitative flow cytometry assay directly performed in whole blood has been used to determine CD11b expression on PMN cells. The results indicate that quantitative flow cytometry can provide consistent CD11b density values between laboratories provided that a calibration system is used including specific calibrators, reagents and protocols. This method allowed us to evidence an up-regulation of CD11b expression for infected patients. This quantitation is a standardized and potentially useful method in clinical situations implying quantitative CD11b expression variations.

Fixation and cryopreservation of whole blood and isolated mononuclear cells: Influence of different procedures on lymphocyte subset analysis by flow cytometry

BACKGROUND

Immunophenotyping of whole blood (WB) and isolated peripheral blood mononuclear cells (PBMCs) is a common tool used to evaluate immune system changes in clinical studies. The development of methods that would allow preservation of samples for flow cytometric analysis is important for the extension of this technology to field testing in settings where the equipment might be not readily accessible.

METHODS

Three-color flow cytometric analysis was used to determine percentages of T cells and their subsets (CD3(+), CD4(+), CD8(+)), B cells (CD19(+)), and natural killer cells (CD16(+)/56(+)) in WB and PBMCs using variations of a standard stain/fix WB staining procedure (Optilyse) that included staining following fixation and freezing of fixed samples before or after staining.

RESULTS

Comparable lymphocyte subset percentages in WB or PBMCs were observed regardless of Optliyse method used (all Ps >/= 0.8). However, differences in fluorescence intensity for several markers were observed across procedures. Compared with the standard stain/fix procedures, fix/stain decreased the mean fluorescence intensities for CD4, CD8, CD19 and CD16/56 in WB and PBMCs (P </= 0.03 for these markers P = 0.105 for CD8 in PBMCs). Further decreases in mean fluorescence intensity were seen with the fix/stain/freeze procedure. The stain/fix/freeze yielded intensities largely comparable to those seen with standard stain/fix procedure (P >/= 0.13), suggesting that, when the markers of interest are known at the time of field collection, implementation of this procedure might be desirable. Fix/freeze/stain resulted in diminution of intensity in general, but they tended to be more modest than those seen for fix/stain/freeze and therefore might be applicable to field studies in instances when the specific markers of interest cannot be defined upfront.

CONCLUSIONS

Freezing of fixed WB and PBMCs before or after cell surface staining is a reliable method for preserving specimens in field sites for later determination of lymphocyte subset percentages, which are commonly assessed in immunodeficient and cancer patients.

Stabilised cellular immuno-fluorescence assay: CD45 expression as a calibration standard for human leukocytes

We first confirmed the precision of the quantitative indirect immunofluorescence (QIFI) test by demonstrating that blood lymphocytes from different individuals expressed CD45 leucocyte antigens at a very similar level (mean: 201 x 10(3) antibody binding capacity (ABC)/lymphocyte) with only minimal variation (CV% 2.5%). These values were maintained for 4 days in blood samples when kept at 20 degrees C, and for up to 14 days in samples fixed with TransFix. Using long-term stabilisation, after an initial drop of 10-15% the CD45 ABC/lymphocyte values remained stable at an 85-90% level for >1 year. These biological standards were used to check other quantitative IF techniques. The quantum simply cellular (QSC) method showed variable results (85-240%), and the QuantiBRITE method gave values as low as 30-40% of the expected values, indicating that biological standards such as CD45 ABC/lymphocytes are absolutely essential to check the performance of methods that claim to quantify immunofluorescence (IF). Next, these standards were used to establish the stabilised cellular immuno-fluorescence assay (SCIFA) as follows. The ABC x 10(3)/cell values established by QIFI on leucocyte populations such as lympho-, mono- and granulocytes were used to create calibration curves for the CD45 antigen and its isoforms CD45RA, -R0 and -RB. The same cell populations were then stained with monoclonal antibodies (MAbs) directly conjugated to different fluorochromes in order to translate the mean fluorescence intensity (MFI) values seen with the directly labelled reagents to values of ABCx10(3)/cells. Using SCIFA with a triple-colour direct IF, the display of CD45 and its isoforms were quantitated on the 'virgin' or 'unprimed' (CD45RA+) and 'primed' (CD45R0+) subsets in both the CD4+ and CD8+ T cell lineages. We also observed that the CD4+ and CD8+ T cells in transit between the 'virgin' and 'primed' subsets frequently displayed different levels of the CD45RA and -R0 molecules, pointing to the physiological variability of the CD45RA-R0 switching process. In conclusion, internal biological standards, with known stable expression of ABC/cell, should be used to evaluate IF staining patterns in a quantitative manner during routine investigations.

Basic principles of flow cytometry

Although the basic principles of flow cytometry have changed little in the past quarter century, the applications of this technology have evolved substantially. As in the past, cytometers interrogate individual cells or particles in a stream with a laser as the cells move past a set of stationary detectors. Increasingly, more colors of fluorescence are being detected by cytometers, faster analysis and sorting rates are becoming possible, cytometers capable of multidirectional sorting are being marketed, and more reagents are becoming available for a wide variety of applications. Furthermore, flow cytometry has not stopped evolving. The development of narrow spectrum flourescent probes, the integration of molecular biologic techniques with flow cytometry, and the evaluation of cell-free markers such as cytokines will be key components in the continuing evolution of flow cytometry.