The measurement of specific cell: cell interactions by dual-parameter flow cytometry

Incompatibility in cell adhesion constitutes a barrier to interspecies chimerism

Interspecies blastocyst complementation holds great potential to address the global shortage of transplantable organs by growing human organs in animals. However, a major challenge in this approach is the limited chimerism of human cells in evolutionarily distant animal hosts due to various xenogeneic barriers. Here, we reveal that human pluripotent stem cells (PSCs) struggle to adhere to animal PSCs. To overcome this barrier, we developed a synthetic biology strategy that leverages nanobody-antigen interactions to enhance interspecies cell adhesion. We engineered cells to express nanobodies and their corresponding antigens on their outer membranes, significantly improving adhesion between different species' PSCs during in vitro assays and increasing the chimerism of human PSCs in mouse embryos. Studying and manipulating interspecies pluripotent cell adhesion will provide valuable insights into cell interaction dynamics during chimera formation and early embryogenesis.

A BAYESIAN MARK INTERACTION MODEL FOR ANALYSIS OF TUMOR PATHOLOGY IMAGES

With the advance of imaging technology, digital pathology imaging of tumor tissue slides is becoming a routine clinical procedure for cancer diagnosis. This process produces massive imaging data that capture histological details in high resolution. Recent developments in deep-learning methods have enabled us to identify and classify individual cells from digital pathology images at large scale. Reliable statistical approaches to model the spatial pattern of cells can provide new insight into tumor progression and shed light on the biological mechanisms of cancer. We consider the problem of modeling spatial correlations among three commonly seen cells observed in tumor pathology images. A novel geostatistical marking model with interpretable underlying parameters is proposed in a Bayesian framework. We use auxiliary variable MCMC algorithms to sample from the posterior distribution with an intractable normalizing constant. We demonstrate how this model-based analysis can lead to sharper inferences than ordinary exploratory analyses, by means of application to three benchmark datasets and a case study on the pathology images of 188 lung cancer patients. The case study shows that the spatial correlation between tumor and stromal cells predicts patient prognosis. This statistical methodology not only presents a new model for characterizing spatial correlations in a multitype spatial point pattern conditioning on the locations of the points, but also provides a new perspective for understanding the role of cell-cell interactions in cancer progression.

Emerging approaches to study cell-cell interactions in tumor microenvironment

Cell-cell interactions are of crucial importance for tissue formation, homeostasis, regeneration processes, and immune response. Recent studies underlined contribution of cell-cell interaction in tumor microenvironment (TME) for tumor progression and metastasis. Cancer cells modify the host cells to tumor-supportive traits, and the modified host cells contribute to tumor progression by interacting with cancer cells and further modifying other normal cells. However, the complex interaction networks of cancer cells and host cells remained largely unknown. Recent advances in high throughput microscopy and single cells-based molecular analyses have unlocked a new era for studying cell-cell interactions in the complex tissue microenvironment at the resolution of a single cell. Here, we review various model systems and emerging experimental approaches that are used to study cell-cell interactions focusing on the studies of TME. We discuss strengths and weaknesses of each model system and each experimental approach, and how upcoming approaches can solve current fundamental questions of cell-cell interactions in TME.

Isolation of HIV-1-reactive antibodies using cell surface-expressed gp160Δc(BaL.)

Significant efforts have been made to identify HIV-1 neutralizing antibodies because they are considered to be critical to the design of an effective HIV-1 vaccine. Although soluble HIV-1 envelope proteins can be used for this purpose, these reagents differ from membrane-anchored HIV-1 envelope spike in a number of important ways and display only a subset of its native epitopes. Consistent with this, some broadly neutralizing antibodies preferentially bind cell surface-expressed HIV-1 envelope, but not the soluble protein. Here we report the details of a new method for isolating anti-HIV-1 specific B cells based on capturing cells that produce antibodies to cell surface-expressed gp160Δc(BaL). While this method is far less efficient than sorting with soluble envelope proteins, it isolated broadly neutralizing anti-HIV-1 antibodies that bind cell surface-expressed gp160Δc(BaL) but not soluble envelope proteins.

Increased effector-target cell conjugate formation due to HLA restricted specific antigen recognition

The T cell receptor (TCR) orchestrates T cell mediated-cytotoxicity through a complex interaction that results in an antigen-specific effector-target cell conjugate formation. While it is well recognized that specific TCR/antigen interactions generate the immunological synapse, their direct contribution to the effector-target cell conjugate has not been conclusively demonstrated. Moreover, since human cytotoxic T lymphocyte (CTL) clones are also susceptible to antigen-independent adhesion to target cells, it remains unclear whether effector-target cell conjugate formation can serve as an indicator of specific antigen recognition by the TCR. To address this question, a well-characterized epitope-specific CTL clone recognizing the melanoma-associated antigen epitope gp100:209-217 in association with HLA-A*0201 was tested against melanoma cell lines lacking or expressing the HLA-A*0201 allele and/or gp100. In this model, TCR/HLA/antigen interactions cooperated with accessory/adhesion molecules to facilitate effector-target cell conjugate formation. HLA-restricted antigen recognition played a dominant role resulting in up to 2-fold increases in conjugate frequency, and a 50% increase of CTL binding to tumor cells over background. The increased number of CTL contained in conjugates correlated with the number of IFN-gamma producing CTL. These results warrant further investigation to evaluate conjugate assays as a potential tool to detect and isolate viable and functionally active CTL. Since conjugate formation analysis does not require knowledge of the target antigen, this assay could potentially be used for enrichment of CTL directed against novel antigens.

Assays of natural killer (NK) cell ligation to target cells

Assays for natural killer cells have long been established in the flow repertoire, but successful application of a flow-based protocol requires attention to many details. This unit provides these details in a comprehensive manner. In particular, the assay is designed for simple bench-top cytometers, rather than for more complex research instruments. Following this protocol, even a novice user should be able to achieve successful completion of an NK assay.

Simultaneous flow cytometric measurement of K-562 megakaryocytic differentiation and CD56+ large granular lymphocyte cytotoxicity

K-562 cells have the capacity to undergo multi-lineage differentiation, which may be crucial to their ability to serve as target reservoirs for CD56+ large granular lymphocytes (LGL). Conventional techniques using chromium release assays to measure lymphocyte-mediated cytotoxicity suffer from disadvantages, including radioactive contamination and the inability to simultaneously determine K-562 and/or CD56+ lymphocyte phenotypes. We illustrate here a three-color flow cytometric method providing for the simultaneous evaluation of K-562-CD56+ LGL binding, K-562 cell viability, and the status of K-562 cell differentiation. Phorbol 12-myristate 13-acetate (PMA) engenders megakaryocytic differentiation in K-562 cell populations, as measured by presentation of the beta(3) integrin (gpIIIa, CD61), while maintaining a negative expression of MHC-I and MHC-II molecules. Using the auto-fluorescence of K-562 cells, flow cytometry can be used to demonstrate a significant decrease in CD56+ LGL activity against K-562 cells in populations pre-incubated with PMA. The capacity of three-color flow cytometry to measure lymphocyte-target cell binding and cell death kinetics, while simultaneously determining target cell phenotype, permits the specific localization of CD61-expressing K-562 cells to areas inconsistent with CD56+ LGL-mediated patterns of lysis.

Measurement of intercellular conjugates by flow cytometry

Analysis of fluorescently labeled cellular aggregates can be accomplished quickly and accurately with the technique of flow cytometry. With a dual-laser flow cytometer, the excitation and emission spectra of fluorophores used to label cells are widely separated, and there is no spillover of emission from one fluorophore into the detector measuring emission from the other. Consequently, conjugates can be measured by a dual-laser instrument when the extent of labeling two cell types with fluorophores are very different. The more commonly available single-laser cytometers can also be used to measure multicellular conjugates, but due to overlaps in emission spectra, the extent of labeling cells with fluorophores must be controlled much more carefully when the single-laser machines are used. This unit describes the labeling of cells and analysis of conjugates with either dual-laser or single laser flow cytometers.

Plug flow cytometry extends analytical capabilities in cell adhesion and receptor pharmacology

BACKGROUND

Plug flow cytometry is a recently developed system for the automated delivery of multiple small boluses or "plugs" of cells or particles to the flow cytometer for analysis. Important system features are that sample plugs are of precisely defined volume and that the sample vessel need not be pressurized. We describe how these features enable direct cell concentration determinations and novel ways to integrate flow cytometers with other analytical instruments.

METHODS

Adhesion assays employed human polymorphonuclear neutrophils (PMNs) loaded with Fura Red and Chinese hamster ovary (CHO) cells cotransfected with genes for green fluorescent protein (GFP) and human P-selectin. U937 cells expressing the human 7-transmembrane formyl peptide receptor were loaded with the fluorescent probe indo-1 for intracellular ionized calcium determinations. A computer-controlled syringe or peristaltic pump loaded the sample into a sample loop of the plug flow coupler, a reciprocating eight-port valve. When the valve position was switched, the plug of sample in the sample loop was transported to the flow cytometer by a pressure-driven fluid line.

RESULTS

In stirred mixtures of PMNs and CHO cells, we used plug flow cytometry to directly quantify changes in concentrations of nonadherent singlet PMNs. This approach enabled accurate quantification of adherent PMNs in multicell aggregates. We constructed a novel plug flow interface between the flow cytometer and a cone-plate viscometer to enable real-time flow cytometric analysis of cell-cell adhesion under conditions of uniform shear. The High Throughput Pharmacology System (HTPS) is an instrument used for automated programming of complex pharmacological cell treatment protocols. It was interfaced via the plug flow coupling device to enable rapid (< 5 min) flow cytometric characterization of the intracellular calcium dose-response profile of U937 cells to formyl peptide.

CONCLUSIONS

By facilitating the coupling of flow cytometers to other fluidics-based analytical instruments, plug flow cytometry has extended analytical capabilities in cell adhesion and pharmacological characterization of receptor-ligand interactions.

Flow cytometric measurement of NK cell immunoconjugates by pulse width processing

Pulse width analysis, in flow cytometry, has been widely used for optimal cell size resolution in cell kinetics analysis. Pulses, generated by scattered light or fluorescence of cells, are electronically analyzed for their height and width. The information generated from these two properties of the pulses is utilized to distinguish signals from single cells vs. signals from cell clumps or aggregates. Pulse width, unlike pulse height, is more sensitive to differences in cell diameter, and therefore can discriminate very small differences in it, which pulse height cannot. We have exploited this property of pulse widths to measure immunoconjugates between NK cells and their targets. Discrimination of the free target cells from the conjugated ones is possible by the pulse widths of only light scatter signals, both forward and/or orthogonal. This resolution was not obtained if pulse height of the same signals was visualized. Using this resolution it was possible to distinguish single cells from the aggregates between target and effector cells. We propose that this is a better method for distinguishing conjugates than the method in which prior vital staining of cells is used.

Subtraction of autofluorescent dead cells from the lymphocyte flow cytometric binding assay

Flow cytometry allows the quantitative analysis of lymphocyte-target cell conjugates and the identification of the lymphocyte subset involved in the binding phenomenon. We recently described a methodology to identify the effector cells bound to K562 targets based on target cell autofluorescence coupled with lymphocyte staining by means of fluorescent monoclonal antibodies. Here we describe an implementation of the methodology that allows the subtraction of spontaneously dead targets to which lymphocytes may or may not adhere, thereby preventing the overestimation of the binding phenomenon and limiting its evaluation to living effector-target conjugates, thus preserving the specificity of the phenomenon.

A multiparameter flow cytometric method to study surface molecules involved in interactions between subpopulations of cells

The interactions between T and B lymphocytes are mediated by several antigen-independent adhesion molecules including LFA-1/ICAM-1 and CD2/LFA-3. Recently new pairs of adhesion molecules involved in T and B interactions have been described: CD28/B7, CD5/CD72 and CD45RO/CD22. In order to study these heterotypic adhesion events, the phenotypes of the subpopulations as well as new potential adhesion molecules involved in conjugate formation, we have developed a flow cytometric method which analyses conjugate formation between T and B cells. The two types of cells were loaded with two vital intracellular dyes: human T lymphocytes purified from blood or tonsils were labelled with BCECF-AM (green fluorescence) and the B lymphoblastoid cell line, RPMI 8866 was labelled with Indo-1-AM (blue fluorescence). The two labelled cell populations were mixed, gently centrifuged for 5 min and then incubated at 37 degrees C in a waterbath for 5 min. The cells were then gently resuspended by inversion and analysed with a double laser flow cytometer. This method permitted us to discover new molecular interactions since preincubation of the two populations with monoclonal antibodies directed against some surface molecules inhibited conjugate formation. As an example, using this technique we found that the low affinity IgE receptor, CD23 and the CR2/EBV receptor are involved in T cell/B cell adhesion and can therefore be considered as a new pair of adhesion molecules. This method also seems to be applicable to recombinant cells bearing a single adhesion molecule such as LFA-1 and ICAM-1. A particular advantage of the two intracellular dyes we used is that they are compatible with the dyes commonly used for classical simultaneous triple colour immunofluorescence (phycoerythrin and Cy-Chrome). We were thus able to determine the subpopulations involved in forming conjugates and we found that T-B conjugates were preferentially formed by CD4, CD45RO positive T cells, which are believed to be the memory T lymphocytes.

Kinetic analysis of cytotoxic lymphocyte-target cell interaction as quantified by dual parameter flow cytometry

The kinetics of conjugate formation between leukemic cell lines (K562 and Daudi) and lymphokine-activated killer (LAK) cells was studied. A flow cytofluorometry method using double immunofluorescence staining was applied. During the first 15 min of incubation of LAK effectors with leukemic targets, a rapid binding occurred, followed by a plateau phase lasting until 30 min of observation. A considerable, yet not statistically significant, between-donor variability was noticed. A mathematical model of conjugate formation kinetics, based on the analogy to enzyme kinetics, was formulated and validated. Parameters of the model were related to the binding capacity of effector and target cells, and to the lifetime of conjugates and free cells. The concordance of theoretical curves with experimental data proved that the described model can be considered as a useful tool for the evaluation of kinetic and dynamic characterization of conjugate formation between leukemic targets and LAK effectors.

Natural killer function in flow cytometry. III. Surface marker determination of K562-conjugated lymphocytes by dual laser flow cytometry

The recognition of effector cell populations that are able to actively from conjugates with target cells is of major importance in studies of lymphocyte cytotoxicity. A number of methodologies have been described to identify the conjugates and count them, but there have been few studies of the binding capability of the different subsets of effector cells involved in the conjugation phenomenon. Here we describe a methodology that permits the study of two surface markers on lymphocytes conjugated to K562 target cells. In particular, the expression of low density CD8 (CD8dim) has been studied on both CD3+ and CD16+ lymphocytes bound to K562 target cells. Previously described methodologies, either optical microscopy or flow cytometry, were not able to identify the effector population by mAb double staining, especially in the case of antigens expressed at low density. The flow cytometric methodology described here permits the measurement of the binding activity of small lymphocyte subsets such as the CD3+ 8dim+ population. However, the method could be used to study the binding activity of any effector population defined by mAb double staining.

New chamber for flow cytometric analysis over an extended range of stream velocity and application to cell adhesion measurements

When analyzed in a flow cytometer, particles are suddenly accelerated to high velocities (1-10 m.s-1) over very short distances. This feature is essential to obtain high analysis rates and low coincidence levels, but translates into very strong velocity gradients (greater than 10(5) s-1): particles experience strong hydrodynamic stresses that elongate them and tend to dissociate weakly associated complexes. In order to analyze fragile conjugates formed by heterotypic adhesion between two cell types, a flow cytometer was modified to make hydrodynamic stress not only much weaker but also adjustable. A new and easily adaptable flow cell was designed for the instruments of the FACS series; it provided satisfactory hydrodynamic conditions on a wide continuous range of flow rates. Accompanying electronic adaptations permitted standard analysis between 0.01 and 10 m.s-1. At 0.01 m.s-1, the velocity gradient roughly amounts to 50 s-1. Conjugates formed by the adhesion between human B and resting T lymphocytes, disrupted in conventional flow cytometers, could be detected and precisely quantified provided analysis velocity was kept below 0.1 m.s-1. We conclude that low velocity flow cytometry makes possible the quantification of weak intercellular adhesion phenomena, and is potentially useful for the future development of new biomechanical techniques and other applications.

Natural killer function in flow cytometry: identification of human lymphoid subsets able to bind to the NK sensitive target K562

NK cells are a phenotypically, morphologically and functionally heterogeneous population. This has led to the current thought that the non-MHC restricted cytotoxicity is a cellular function that can be associated to different phenotypes. The recognition of the target cell and the conjugate formation is always the first step that eventually leads to the lysis of target. Characterization of the phenotypical pattern of the cells able to bind to K562 targets is the purpose of this study. A multi-parametric flow cytometry binding assay has been employed to identify the different K562-bound lymphocyte subsets. In particular, cells that coexpress the CD16 and CD8 antigens (CD16+8dim+) showed a significantly higher binding capacity than their CD16+8- counterpart. Moreover, the highest binding values have been found in cells that did not express the CD16 antigen at all, but still expressed the CD8dim antigen, such as the small CD8dim+3+ population. These data show that, the NK lytic function being dependent on binding, minor subpopulations must be considered among effector cells, which might correspond to different lytic activities. None of the previously published methodologies that analyze conjugates by flow cytometry or fluorescence microscopy were able to measure the binding capacity of small, double stained, lymphocyte subsets.

Specific binding and lysis of human melanoma by IL-2-activated cells coated with anti-T3 or anti-Fc receptor cross-linked to antimelanoma antibody: a possible approach to the immunotherapy of human tumors

Monoclonal antibodies (MoAb) to human melanoma have demonstrated a limited ability to cause tumor regression in humans when used alone or when coupled to gamma-emitting radioisotopes. We have evaluated heteroaggregates containing antilymphocyte antibodies crosslinked to antimelanoma monoclonal antibodies recognizing p97, a transferrin-like molecule (MoAb 96.5). When coupled to antibodies recognizing T3 (CD3, part of the T-cell receptor complex for antigen) or to 3G8, an antibody recognizing the Fc receptor present on large granular lymphocytes and granulocytes (CD16), significant induction of effector target crosslinks and target cell lysis could be obtained. Effector cells incubated for 24 hr with recombinant IL-2 were coated with the crosslinked reagents and tested for conjugate formation and for cytotoxicity in a 4-hr assay with chromium-labeled targets. Marked increases in conjugation to autologous tumor (47.0% compared to 11.8%) was demonstrated with E+ cells using the T3-coupled MoAb and with E- cells using the Fc receptor-coupled MoAb (22.6% compared to 11.2%). When tested in sequential cytotoxicity assays using unseparated effector cells incubated for 1, 2, and 3 days in IL-2, lytic activity was less than 2, less than 2, and 3.3 LU/10(6) cells for cells incubated in monomeric 96.5; 2.6, 5.3, and 50 LU/10(6) cells incubated in 96.5 crosslinked T3; and less than 2, 3.6, and 8.0 LU/10(6) cells for cells incubated with 96.5 crosslinked to 3G8. Similar findings were noted in two other experiments. Heteroaggregates such as these may be useful in conjunction with the transfer of IL-2-activated cells or with IL-2 alone in immunotherapy trials.

Ia+ murine epidermal Langerhans cells are deficient in surface expression of the class I major histocompatibility complex

Murine epidermal Langerhans cells were analyzed with fluorescence microscopy and multicolor flow cytometry for the surface expression of major histocompatibility complex (MHC) class I and class II antigens. Langerhans cells of H-2k haplotype were identified in situ or in epidermal-cell suspensions by their surface expression of the MHC class II determinants I-Ak and I-Ek. More than 90% of class II-positive Langerhans cells in epidermal-cell suspensions expressed no or barely detectable amounts of MHC class I antigens. Quantitation by flow cytometry revealed that H-2k Langerhans cells expressed only 1.6-3.3% as much H-2Kk as did class II-negative keratinocytes in the same epidermal-cell suspensions. By fluorescence microscopy, class I MHC antigens were not detectable on Langerhans cells in situ when analyzed on sheets of intact epidermis. The deficient expression of class I MHC permitted highly purified Langerhans cell populations to be isolated from epidermal cell suspensions by treatment with anti-class I MHC monoclonal antibody and complement. It is likely that the uniquely low cell-surface expression of class I MHC antigen by Langerhans cells has relevance to both immune responses in the skin as well as to mechanisms of skin allograft rejection. In addition, it is conceivable that regulation of class I MHC expression on antigen-presenting cells in general is an important but hitherto unrecognized mechanism of immune regulation.

Two antigen-independent adhesion pathways used by human cytotoxic T-cell clones

Cell-cell adhesion is essential for many immunological functions, including interaction of cytotoxic T lymphocytes (CTLs) with their targets. We have explored CTL-target interactions using well-characterized cloned human CTLs. Conjugate formation between these CTLs and many antigen-negative targets is almost as efficient as with specific target cells, but does not lead to target-cell lysis. Thus, on specific target cells, adhesion by antigen-independent pathways may occur concurrently with or precede antigen recognition. The molecules LFA-1, CD2 (T11, LFA-2) and LFA-3 have been shown to be involved in human CTL conjugation with and lysis of specific target cells. Here we describe monoclonal antibody inhibition studies using individual monoclonal antibodies and mixes which demonstrate (1) that LFA-1, CD2 and LFA-3 are involved in antigen-independent conjugate formation; and (2) suggest that CD2 and LFA-3 are involved in one pathway and LFA-1 in another. We confirmed the existence of distinct pathways by the demonstration that LFA-1-dependent adhesion requires divalent cations and is temperature-sensitive whereas CD2- and LFA-3-dependent adhesion does not require divalent cations and is temperature-insensitive. Together with previous data, our studies suggest that CD2 on the effector interacts with LFA-3 as its ligand on targets.

Quantitation and sorting of vitally stained natural killer cell-target cell conjugates by dual beam flow cytometry

We have detected formation of stable associations, or conjugates, between fluorescein diacetate-(FDA) stained human natural killer (NK) cells and Hoechst 33342-(HO342) stained tumor cells by dual laser flow cytometry. Conjugates in mixtures of effectors and targets emitted both green (FDA) and blue (HO342) fluorescence. This was confirmed by cell sorting. More than 90% of the conjugates included one target and one effector cell. Conjugate formation frequency was temperature independent between 4 and 37 degrees C, optimized by 10 min, and stable for 1 hr. Enrichment of effector populations for cells mediating lysis of standard NK targets and for cells reacting with OKM1, Leu-7, and Leu-11b monoclonal antibodies also enriched conjugate-forming cells. Lysis of either OKM1+, Leu-11b+ effector subpopulations with antibody and complement eliminated, but treatment with these antibodies alone had no effect on, conjugate formation. Effector pretreatment with Leu-4 or 3A1 and complement increased the frequency of conjugation slightly. Flow-determined frequencies of NK-conjugate formation with 14 target cell lines correlated well with data derived from standard microscopic assays. However, the flow method was more rapid, could be used when target and effector were of comparable size, and permitted isolation of conjugates by sorting.

Production of target-specific effector cells using hetero-cross-linked aggregates containing anti-target cell and anti-Fc gamma receptor antibodies

Rabbit anti-2,4-dintrophenyl (DNP) antibodies or their F(ab')2 fragments were chemically cross-linked to the anti-mouse Fc gamma R monoclonal antibody 2.4G2 or to its Fab fragment. P388D1 cells were incubated with heteroaggregates between 2.4G2 and anti-DNP (anti-Fc gamma R X anti-DNP) and washed. The resulting cells lysed 2,4,6-trinitrophenyl chicken erythrocytes (TNP CRBC) in a hapten-specific manner. The lysis was inhibited by free hapten but was resistant to inhibition by immune complexes. Other cells coated with antibody heteroaggregates also mediated lysis of TNP-modified target cells. For example, mouse resident peritoneal exudate cells (PEC) lysed TNP CRBC and bacillus Calmette-Guérin-activated PEC lysed both TNP CRBC and TNP tumor targets. Human neutrophils, when incubated with heteroaggregates containing the anti-human neutrophil Fc gamma R antibody 3G8 and anti-DNP also lysed TNP CRBC and TNP-modified tumor cells. To test whether linkage to Fc gamma R was required for lysis, F(ab')2 fragments from the anti-KdDd monoclonal antibody 34-1-2 were cross-linked to anti-DNP F(ab')2 fragments. P388D1 cells (which express Kd and Dd) were then incubated with these heteroaggregates and washed, and their abilities to form conjugates and lyse TNP CRBC were compared with those of P388D1 cells treated with anti-Fc gamma R X anti-DNP. In both cases, P388D1 cells formed conjugates. However, only the cells treated with anti-Fc gamma R X anti-DNP mediated lysis to a significant extent. We conclude that heteroaggregates containing anti-Fc gamma R and anti-target cell antibodies can be used to create potent effector cells against red cell and tumor targets and that bridging of effectors with target cells directly to Fc gamma R on effector cells is required for lysis.