Abstract 3541: A method to visualize and quantify effects of immunotherapies in a physiologically relevant 3D TME model

INTRODUCTION: In vitro assays based on 3D co-cultures of cancer and immune cells present an opportunity to test diverse immunotherapies and understand their mechanisms of action in boosting anti-tumor immune responses in cancer patients. Incorporation of diverse cellular players, including cancer associated fibroblasts (CAFs) and suppressive myeloid cell populations, allows for faithful recapitulation of complex cellular interactions that occur in the tumor micro-environment (TME). The application of automation results in a high throughput platform which combines physiological relevance with assay robustness and reproducibility. Functional read-outs, such as migration of immune cells towards tumoroids, infiltration into the tumoroids and their killing are obtained by applying high content imaging, allowing for better understanding of the immune-modulatory profile of IO drugs. Here, we have applied image analysis to quantify the effects of different antibodies and small molecules targeting both the tumor and the T cells.

MATERIAL and METHODS: Tumoroids generated from breast and colon cancer organoids and cell lines were cultured in protein hydrogel. Partially HLA-matching T cells isolated from healthy PBMCs donors were added with or without activation to the 3D culture after incorporation of different suppressive populations, including CAFs and M2 macrophages. The capacity of effector T cells to infiltrate and subsequently kill tumor structures was visualized using high-content microscopy and quantified with morphometric image analysis software after reconstitution of 3D image stacks.

RESULTS and DISCUSSION: Automated 3D image and data analysis enabled discrimination of cellular interactions in a complex TME model. Depending on the activation status of T cells and the presence of suppressive cells in the environment, different levels of infiltration and tumor killing by effector T cells were measured. The suppression induced on the T cells by the environment was partially reversed by immune-modulators (e.g. CSF1R inhibitor) added to the co-culture, which was confirmed by increased IFNγ levels in culture supernatants. Two quantitative read outs that are strictly dependent on the 3D environment; T cell infiltration into tumor and decrease in tumor volume were combined to demonstrate immune suppression release induced by different immuno-modulators.

CONCLUSION: Our image-based platform described here allows for analysis of immunotherapy effects on cells that engage in a physiologically relevant spatial setting and in the presence of suppressive TME elements. Visualization and quantification of these complex cellular interactions within the TME empowers immunotherapy drug developers with a highly powerful tool to select the most promising candidates and better understand their mechanism of action, which ultimately will have an impact in the clinic.

Citation Format: Lidia Daszkiewicz, Gera Goverse, Nataliia Beztsinna, Saskia de Man, Tomas Veenendaal, Michelle Klop, Daniel Okkes, Ashgard Weterings, Kuan Yan, Leo Price. A method to visualize and quantify effects of immunotherapies in a physiologically relevant 3D TME model [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 3541.

Abstract P113: Image-based quantification of immunotherapeutic effect on the tumor-immune interactions in 3D co-cultures

INTRODUCTION Despite the increasing number of immunotherapies available in preclinical cancer research and drug discovery pipelines, the number of patients benefiting from these immunotherapies remains extremely low. Moreover, the failure of immunotherapies in clinical trials remains very high to date. These high failure rates could be attributed to the lack of deep understanding the mechanism of action of drug candidates and to the absence of clinically relevant preclinical models that are suitable for drug screening purposes. To address these issues, we have developed an in vitro drug screening platform suitable for high-throughput testing in physiologically relevant 3D environment. Our preclinical drug testing platform is based on the co-culture of tumor cells with immune cells in 3D. Image analysis is applied to read out effects of drug candidates on immune cell migration, immune cell infiltration and tumoroid killing. MATERIALS AND METHODS 3D tumor cultures from human cancer cell lines (including breast, lung and colon cancer) or patient-derived colorectal organoids (from HUB Organoid Technology) were generated by embedding tumor cells in 3D ECM-like matrix constituted from protein hydrogel. Subsequently, different immune subsets, including PBMCs from healthy donors, T cells, NK cells or macrophages, were stained with cell tracker and added to the cultures. The cultures were treated with different immune-modulators (e.g. superantigens, activating antibodies, T cell engagers, CSFR1 inhibitor or STING agonist). The effect of immune-modulators on immune cell infiltration and their killing was assessed by high-content imaging and quantified after morphometric analysis with the proprietary Ominer® software. Immune cell-mediated killing was confirmed by measurement of IFNγ secretion in the culture supernatants. RESULTS Image-based analysis allowed for the dissection of complex tumor-immune cell interactions in the 3D cultures. Moreover, morphometric analysis revealed different levels of immune cell infiltration and tumoroid killing upon treatment with different immune-modulators. These effects could be reduced by inhibitory signals from the tumor microenvironment (TME) and were confirmed by the levels of IFNγ secreted by the immune cells. CONCLUSION Our in vitro platform allows quantitative image-based analysis of 3D tumor-immune cell co-cultures in a high-throughput manner, based on spatially resolved information in a more physiologically-relevant setting compared to traditional 2D cultures. This image-based analysis could be employed to dissect the effect of immunotherapy on different cell populations with the TME representing a promising tool to improve our understanding of the mechanism of action of novel treatments, treatment relapse and combination strategies, to eventually lead to a better clinical performance.

Citation Format: Lidia Daszkiewicz, Gera Goverse, Nataliia Beztsinna, Marjan van de Merbel, Benjamin Visser, Tomas Veenendaal, Emma Spanjaard, Kuan Yan, Leo Price. Image-based quantification of immunotherapeutic effect on the tumor-immune interactions in 3D co-cultures [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2021 Oct 7-10. Philadelphia (PA): AACR; Mol Cancer Ther 2021;20(12 Suppl):Abstract nr P113.

Abstract P158: Phenotypic analysis of myeloid cells in a 3D image-based repolarization assay with tumor spheroids

Background The immunosuppressive tumor microenvironment (TME) involves multiple cell types and a better understanding of the interplay between these cells could potentially unleash the full potential of many different types of immunotherapies. Tumor infiltrating myeloid cells have both cancer-restraining and cancer-promoting functions. Therefore, to further increase the biological relevance of in vitro platform, we incorporated the myeloid cell compartment into 3D co-cultures of tumor cells and T cells to measure the effects of immune-modulators. Using our proprietary image analysis software and machine learning, a set of morphological features was identified that allowed discrimination between undifferentiated monocytes, M1 and M2 macrophages, and dendritic cells. In addition, the phenotypic profiles of the myeloid cells could be analyzed in the presence of the tumor supernatants or in co-cultures with tumor cells. This assay allows a better understanding of the suppressive tumor microenvironment including multiple cell types and is suitable to test different cancer immunotherapies. Materials and Methods Different myeloid cell populations were generated from healthy PBMCs. Polarized M1 and M2 macrophages, DCs, and undifferentiated monocytes were then co-cultured with tumor conditioned media, spheroids derived from different cancer cell lines, or colorectal cancer organoids (CRC), growing in protein hydrogel in 384 well-plates for 1-7 days. In addition, purified T cells were also incorporated in these cultures. The cellular interactions were visualized using high-content microscopy and OMiner® software, which was trained to identify phenotypic profiles of different myeloid cell populations in 3D, was used to quantify their similarity to the defined subsets upon treatment. Results Myeloid cell populations were classified according to their phenotypic features identified by 3D image analysis, which verified the repolarization of M2 macrophages by their shift into phenotypic space of the M1 type macrophages upon treatment with CSF1r inhibitor or STING agonist. Repolarization was confirmed with an increased similarity score towards M1 macrophages upon treatment. This approach was further used to reveal the different effects of tumor cells and their immunosuppressive TME on myeloid cell phenotypes, showing that each tumor differently influenced the repolarization of the myeloid cells. In addition, triple co-cultures with T cells revealed the suppressive effect of tumor associated myeloid cells on the proliferation and infiltration of T cells. Conclusions The phenotypic analysis of different myeloid cells in 3D co-cultures could be visualized and quantified elucidating the bi-directional interplay between tumor and immune cells, and the functional reprograming of the suppressive tumor associated population towards an M1 phenotype induced by drug candidates. This advanced platform for testing cancer immunotherapies combines the ability to examine the complexity of the TME with the robustness of a high-throughput screening platform.

Citation Format: Gera Goverse, Nataliia Beztsinna, Benjamin Visser, Tomas Veenendaal, Marjan van de Merbel, Emma Spanjaard, Ashgard Weterings, Kuan Yan, Leo Price, Lidia Daszkiewicz. Phenotypic analysis of myeloid cells in a 3D image-based repolarization assay with tumor spheroids [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2021 Oct 7-10. Philadelphia (PA): AACR; Mol Cancer Ther 2021;20(12 Suppl):Abstract nr P158.

Characterization of different oligomeric forms of CRISP2 in the perinuclear theca versus the fibrous tail structures of boar spermatozoa

Mammalian sperm carry a variety of highly condensed insoluble protein structures such as the perinuclear theca, the fibrous sheath and the outer dense fibers, which are essential to sperm function. We studied the role of cysteine rich secretory protein 2 (CRISP2); a known inducer of non-pathological protein amyloids, in pig sperm with a variety of techniques. CRISP2, which is synthesized during spermatogenesis, was localized by confocal immunofluorescent imaging in the tail and in the post-acrosomal region of the sperm head. High resolution localization by immunogold labeling electron microscopy (EM) of ultrathin cryosections revealed that CRISP2 was present in the perinuclear theca and neck region of the sperm head, as well as in the outer dense fibers and the fibrous sheath of the sperm tail. Interestingly, we found that under native, non-reducing conditions CRISP2 formed oligomers both in the tail and the head but with different molecular weights and different biochemical properties. The tail oligomers were insensitive to reducing conditions but nearly complete dissociated into monomers under 8 M urea treatment, while the head 250 kDa CRISP2 positive oligomer completely dissociated into CRISP2 monomers under reducing conditions. The head specific dissociation of CRISP2 oligomer is likely a result of the reduction of various sulfhydryl groups in the cysteine rich domain of this protein. The sperm head CRISP2 shared typical solubilization characteristics with other perinuclear theca proteins as was shown with sequential detergent and salt treatments. Thus, CRISP2 is likely to participate in the formation of functional protein complexes in both the sperm tail and sperm head, but with differing oligomeric organization and biochemical properties. Future studies will be devoted to the understand the role of CRISP2 in sperm protein complexes formation and how this contributes to the fertilization processes.

Synthetic Extracellular Matrices as a Toolbox to Tune Stem Cell Secretome

The application of stem cell-derived secretome in regenerative therapies offers the key advantage that instead of the stem cells, only their effective paracrine compounds are in vivo delivered. Ideally, the secretome can be steered by the culture conditions of the stem cells. So far, most studies use stem cells cultured on stiff plastic substrates, not representative of their native 3D environment. In this study, cells are cultured inside synthetic polyisocyanide (PIC)-based hydrogels, which are minimal, tailorable, and highly reproducible biomimetic matrices. Secretome analysis of human adipose-derived stem cells (multiplex, ELISA) displays that matrix manipulation is a powerful tool to direct the secretome composition. As an example, cells in nonadherent PIC gels secrete increased levels of IL-10 and the conditioned media from 3D culture accelerate wound closure. In all, our PIC-based approach opens the door to dedicated matrix design to engineer the secretome for custom applications.

Characterization of Endothelial and Smooth Muscle Cells From Different Canine Vessels

Vasculature performs a critical function in tissue homeostasis, supply of oxygen and nutrients, and the removal of metabolic waste products. Vascular problems are implicated in a large variety of pathologies and accurate in vitro models resembling native vasculature are of great importance. Unfortunately, existing in vitro models do not sufficiently reflect their in vivo counterpart. The complexity of vasculature requires the examination of multiple cell types including endothelial cells (ECs) and vascular smooth muscle cells (VSMCs), as well as vessel location in the body from which they originate. The use of canine blood vessels provides a way to study vasculature with similar vessel size and physiology compared to human vasculature. We report an isolation procedure that provides the possibility to isolate both the endothelial and smooth muscle cells from the same vessels simultaneously, enabling new opportunities in investigating vasculature behavior. Canine primary ECs and VSMCs were isolated from the vena cava, vena porta and aorta. All tissue sources were derived from three donors for accurate comparison and to reduce inter-animal variation. The isolation and purification of the two distinct cell types was confirmed by morphology, gene- and protein-expression and function. As both cell types can be derived from the same vessel, this approach allows accurate modeling of vascular diseases and can also be used more widely, for example, in vascular bioreactors and tissue engineering designs. Additionally, we identified several new genes that were highly expressed in canine ECs, which may become candidate genes for novel EC markers. In addition, we observed transcriptional and functional differences between arterial- and venous-derived endothelium. Further exploration of the transcriptome and physiology of arteriovenous differentiation of primary cells may have important implications for a better understanding of the fundamental behavior of the vasculature and pathogenesis of vascular disease.

Vps3 and Vps8 control integrin trafficking from early to recycling endosomes and regulate integrin-dependent functions

Recycling endosomes maintain plasma membrane homeostasis and are important for cell polarity, migration, and cytokinesis. Yet, the molecular machineries that drive endocytic recycling remain largely unclear. The CORVET complex is a multi-subunit tether required for fusion between early endosomes. Here we show that the CORVET-specific subunits Vps3 and Vps8 also regulate vesicular transport from early to recycling endosomes. Vps3 and Vps8 localise to Rab4-positive recycling vesicles and co-localise with the CHEVI complex on Rab11-positive recycling endosomes. Depletion of Vps3 or Vps8 does not affect transferrin recycling, but delays the delivery of internalised integrins to recycling endosomes and their subsequent return to the plasma membrane. Consequently, Vps3/8 depletion results in defects in integrin-dependent cell adhesion and spreading, focal adhesion formation, and cell migration. These data reveal a role for Vps3 and Vps8 in a specialised recycling pathway important for integrin trafficking.

Extent of linkage disequilibrium in chicken

Many of the economically important traits in chicken are multifactorial and governed by multiple genes located at different quantitative trait loci (QTLs). The optimal marker density to identify these QTLs in linkage and association studies is largely determined by the extent of linkage disequilibrium (LD) around them. In this study, we investigated the extent of LD on two chromosomes in a white layer and two broiler chicken breeds. Pairwise levels of LD were calculated for 33 and 36 markers on chromosomes 10 and 28, respectively. We found that useful LD (i.e. an r(2) value higher than 0.3) in Nutreco chicken breed E5 (inbred) can extend to around 1 cM on chromosomes 10 and 28, although in a second region on chromosome 28 it extends to about 2.5 cM. The extent in breed Nutreco E3 (outbred) was very short in chromosome 10 (15 kb) but very much larger on chromosome 28, particularly in one region of depressed heterozygosity. The layer breed E2 (inbred) showed an extent of useful LD up to 4 cM on chromosome 10; the extent on chromosome 28 could not be assessed due to an erratic pattern of LD on that chromosome, although in one region LD appears to be in the order of 0.8 cM. This indicates that there may be very large differences in patterns of LD between different chicken breeds and different genomic regions.

Genetic mapping of quantitative trait loci affecting susceptibility in chicken to develop pulmonary hypertension syndrome

Pulmonary hypertension syndrome (PHS), also referred to as ascites syndrome, is a growth-related disorder of chickens frequently observed in fast-growing broilers with insufficient pulmonary vascular capacity at low temperature and/or at high altitude. A cross between two genetically different broiler dam lines that originated from the White Plymouth Rock breed was used to produce a three-generation population. This population was used for the detection and localization of quantitative trait loci (QTL) affecting PHS-related traits. Ten full-sib families consisting of 456 G2 birds were typed with 420 microsatellite markers covering 24 autosomal chromosomes. Phenotypic observations were collected on 4202 G3 birds and a full-sib across family regression interval mapping approach was used to identify QTL. There was statistical evidence for QTL on chicken chromosome 2 (GGA2), GGA4 and GGA6. Suggestive QTL were found on chromosomes 5, 8, 10, 27 and 28. The most significant QTL were located on GGA2 for right and total ventricular weight as percentage of body weight (%RV and %TV respectively). A related trait, the ratio of right ventricular weight as percentage to total ventricular weight (RATIO), reached the suggestive threshold on this chromosome. All three QTL effects identified on GGA2 had their maximum test statistic in the region flanked by markers MCW0185 and MCW0245 (335-421 cM).

Chromosomal assignment of chicken clone contigs by extending the consensus linkage map

The bacterial artificial clone-based physical map for chicken plays an important role in the integration of the consensus linkage map and the whole-genome shotgun sequence. It also provides a valuable resource for clone selection within applications such as fluorescent in situ hybridization and positional cloning. However, a substantial number of clone contigs have not yet been assigned to a chromosomal location or have an ambiguous chromosome assignment. In this study, 86 single nucleotide polymorphism markers derived from 86 clones were mapped on the genetic map. These markers added anchoring information for 56 clone contigs and 13 individual clones, covering a total of 57,145 clones.

Integration of chicken genomic resources to enable whole-genome sequencing

Different genomic resources in chicken were integrated through the Wageningen chicken BAC library. First, a BAC anchor map was created by screening this library with two sets of markers: microsatellite markers from the consensus linkage map and markers created from BAC end sequencing in chromosome walking experiments. Second, HINdIII digestion fingerprints were created for all BACs of the Wageningen chicken BAC library. Third, cytogenetic positions of BACs were assigned by FISH. These integrated resources will facilitate further chromosome-walking experiments and whole-genome sequencing.

The gene orders on human chromosome 15 and chicken chromosome 10 reveal multiple inter- and intrachromosomal rearrangements

Comparative mapping between the human and chicken genomes has revealed a striking conservation of synteny between the genomes of these two species, but the results have been based on low-resolution comparative maps. To address this conserved synteny in much more detail, a high-resolution human-chicken comparative map was constructed from human chromosome 15. Mapping, sequencing, and ordering of specific chicken bacterial artificial chromosomes has improved the comparative map of chromosome 15 (Hsa15) and the homologous regions in chicken with almost 100 new genes and/or expressed sequence tags. A comparison of Hsa15 with chicken identified seven conserved chromosomal segments between the two species. In chicken, these were on chromosome 1 (Gga1; two segments), Gga5 (two segments), and Gga10 (three segments). Although four conserved segments were also observed between Hsa15 and mouse, only one of the underlying rearrangement breakpoints was located at the same position as in chicken, indicating that the rearrangements generating the other three breakpoints occurred after the divergence of the rodent and the primate lineages. A high-resolution comparison of Gga10 with Hsa15 identified 19 conserved blocks, indicating the presence of at least 16 intrachromosomal rearrangement breakpoints in the bird lineage after the separation of birds and mammals. These results improve our knowledge of the evolution and dynamics of the vertebrate genomes and will aid in the clarification of the mechanisms that underlie the differentiation between the vertebrate species.

Polymorphic microsatellites developed by cross-species amplifications in common pheasant breeds

Genetic variability was analysed in two common breeds of pheasant (Phasianus colchicus L. 1758) by means of cross-species amplifications of microsatellite loci: 154 chicken, Gallus gallus and 32 turkey, Meleagris gallopavo, primers were tested for amplification of pheasant DNA. Thirty-six primers (25 specific for chicken and 11 for turkey) amplified pheasant DNA. Fifteen markers yielded specific products and were tested for polymorphism. Eight of them (55%) were polymorphic, with an average polymorphism of two alleles per locus. Specific polymerase chain reaction (PCR) products were sequenced; repeats were found in 11 of the 15 markers, although only two loci showed the same repeat and could be homologous to chicken ones.

Differences in the endosomal distributions of the two mannose 6-phosphate receptors

Multiple immunolabeling of cryosections was performed to compare the subcellular distributions of the two mannose 6-phosphate receptors (MPRs) involved in the intracellular targeting of lysosomal enzymes: the cation-dependent (CD) and cation-independent (CI) MPR. In two cell types, the human hepatoma cell line HepG2 and BHK cells double transfected with cDNA's encoding for the human CD-MPR and CI-MPR, we found the two receptors at the same sites: the trans-Golgi reticulum (TGR), endosomes, electron-dense cytoplasmic vesicles, and the plasma membrane. In the TGR the two receptors colocalized and were concentrated to the same extent in the same HA I-adaptor positive coated buds and vesicles. Endosomes were identified by the presence of exogenous tracers. The two MPR codistributed to the same endosomes, but semiquantitative analysis showed a relative enrichment of the CI-MPR in endosomes containing many internal vesicles. Two endosomal subcompartments were discerned, the central vacuole and the associated tubules and vesicles (ATV). We found an enrichment of CD-MPR over CI-MPR in the ATV. Lateral segregation of the two receptors within the plane of membranes was also detected on isolated organelles. Double immunolabeling for the CD-MPR and the asialoglycoprotein receptor, which mainly recycles between endosomes and the plasma membrane, revealed that these two receptors were concentrated in different subpopulations of endosomal ATV. The small GTP-binding protein rab4, which has been shown to mediate recycling from endosomes to the plasma membrane, was localized at the cytosolic face of many endosomal ATV. Quantitative analysis of double-immunolabeled cells revealed only a limited codistribution of the MPRs and rab4 in ATV. These data suggest that the two MPRs exit the TGR via the same coated vesicles, but that upon arrival in the endosomes CD-MPR is more rapidly than CI-MPR, segregated into ATV which probably are destined to recycle MPRs to TGR.

Intracellular processing of pulmonary surfactant protein B in an endosomal/lysosomal compartment

The hydrophobic surfactant protein B (SP-B) is synthesized in alveolar type II cells as a 40-kDa precursor protein that is processed via 39- and 23-kDa intermediates to the mature 8-kDa size. To determine the site of SP-B processing, the subcellular distribution of the precursor and mature forms of SP-B was investigated on ultrathin cryosections of human lung using two polyclonal antibodies that discriminate between precursor forms and the mature form of SP-B. An antibody against the human lysosomal membrane glycoprotein CD63 was used to identify organelles in the endosomal/lysosomal pathway. Precursor SP-B was present in the endoplasmic reticulum, the Golgi complex, and multivesicular bodies but was absent from lamellar bodies and plasma membrane. Mature SP-B was present in multivesicular bodies and lamellar bodies but almost completely absent in the endoplasmic reticulum and the Golgi complex. Semiquantitative evaluation of the immunogold labeling by counting the gold particles representing precursor or mature SP-B in the different compartments showed that the mature-to-precursor ratio was low in the endoplasmic reticulum and the Golgi complex (0.13 and 0.11, respectively), increased in the multivesicular bodies to 3.4, and was very high (65) in lamellar bodies. Multivesicular bodies and lamellar bodies contain the lysosomal membrane marker CD63 and are therefore part of the lysosomal pathway. These data strongly suggest that precursor SP-B is proteolytically processed to its mature 8-kDa form intracellularly in an endosomal/lysosomal compartment, most probably in multivesicular bodies.

Immunocytochemical localization of surfactant protein D (SP-D) in type II cells, Clara cells, and alveolar macrophages of rat lung

We investigated the cellular and subcellular distribution of surfactant protein D (SP-D) by immunogold labeling in lungs of adult rats that had been given bovine serum albumin coupled to 5-nm gold (BSAG) for 2 hr to visualize the endocytotic pathway. Specific gold labeling for SP-D was found in alveolar Type II cells, Clara cells, and alveolar macrophages. In Type II cells abundant labeling was observed in the endoplasmic reticulum, whereas the Golgi complex and multivesicular bodies were labeled to a limited extent only. Lamellar bodies did not seem to contain SP-D. Gold labeling in alveolar macrophages was restricted to structures containing endocytosed BSAG. In Clara cells labeling was found in the endoplasmic reticulum, the Golgi complex, and was most prominent in granules present in the apical domain of the cell. Double labeling experiments with anti-surfactant protein A (SP-A) showed that both SP-A and SP-D were present in the same granules. However, SP-A was distributed throughout the granule contents, whereas SP-D was confined to the periphery of the granule. The Clara cell granules are considered secretory granules and not lysosomes, because they were not labeled for the lysosomal markers cathepsin D and LGP120, and they did not contain endocytosed BSAG.

Surfactant protein A is localized at the corners of the pulmonary tubular myelin lattice

Immunogold labeling on sections of a freeze-substituted tubular myelin-enriched fraction isolated from a bronchoalveolar lavage of rat lung showed that surfactant protein A (SP-A) occurs predominantly at the corners of the tubular myelin lattice. Seventy-nine percent of the gold particles were located within 20 nm from a corner. Extracellular SP-A was detected only in the tubular myelin lattice and not in vesicles or secreted lamellar bodies. Ultra-thin cryosections of rat lung fixed in vivo showed that intracellular SP-A was distributed homogeneously over the stacked membranes of lamellar bodies in alveolar Type II cells. The presence of SP-A at the corners of the tubular myelin lattice suggests an important role of this protein in the formation and/or maintenance of this highly ordered lattice.

Immunogold determination of amylase concentrations in pancreatic subcellular compartments

We used the immunogold method on ultrathin cryosections to measure intracellular amylase (Am) concentrations in subcellular compartments of rat exocrine pancreatic cells. Previously, the quantitation procedure was characterized in a model system consisting of Am dispersed at known concentration in a matrix of gelatin. Variations in labeling efficiency, due to differences in matrix density, were equalized by embedding in 30% polyacrylamide (PAA). Here we applied these model conditions to rat pancreas and established intracellular Am concentrations [Am]. Specimen blocks were composed of tissue and a reference layer of gelatin mixed with a known Am concentration ([Am]r), both fixed in glutaraldehyde. Cryosections of the PAA embedded blocks were immunogold labeled for Am. The labeling density was measured in the reference layer (LDr) and in structures in exocrine cells that were involved in Am synthesis and transport (LDs). In each of these structures the Am concentration ([Am]s) was calculated from: [Am]s = [Am]r. LDs/LDr In this way we measured average concentrations ranging from 63 mg/ml in rough endoplasmic reticulum to 261 mg/ml in secretory granules. Concentration of Am appeared to occur mainly in the most cis- and the most trans-Golgi cisternae. To check whether sterical hindrance was an inherent bias to the [Am] measurements in compartments that contained high concentrations of the enzyme, the labeling efficiency for Am in intact isolated secretory granules in gelatin and embedded in PAA, was compared with the efficiency when the granules were lysed and approximately 50 times diluted in gelatin before PAA embedment. It appeared that Am was detected with similar efficiency under both conditions. This demonstrated that sterical hindrance did not cause errors in the measurements of cellular Am concentrations.