Evidence for a trans-acting activator function regulating the expression of the human CD5 antigen

Interspecies somatic cell hybrids were generated by fusing the mouse T-lymphoma cell line, BW5147, with normal human T lymphocytes at different stages of differentiation. Thymocytes, activated peripheral T lymphocytes, or an activated T-cell clone were used as human partners, respectively, in three independent fusions. Irrespective of the human cell partner used for fusion, a certain number of hybrids lost CD5 surface expression over a period of time in culture. Analysis at the phenotype and genetic level showed that lack of CD5 expression was due neither to segregation of human autosome 11, on which the CD5 gene has been mapped, nor to deletion of the CD5 structural gene. Furthermore, loss of CD5 surface expression correlated with the absence of specific mRNA. Since these hybrids preferentially segregate human chromosomes, these results indicate the existence of a non-syntenic trans-active locus, or loci, positively controlling the expression of the human CD5 gene.

Comparison of four in vitro assays for specific IgE detection

Three immunoenzymatic techniques for specific IgE detection (Pharmacia CAP System, Kallestad Allercoat System, Neo Abellò Hamlet-IgE) and the classical Phadebas RAST were compared using 34 sera from patients with a clinical diagnosis of allergic disease and 19 sera from healthy non-atopic controls. IgE antibodies to 9 aeroallergens and 6 food antigens were assessed and 399 tests were run with each method. All techniques showed a high specificity (92%-100%) and satisfactory efficiency (82%-98%), while the sensitivity for RAST, CAP, Allercoat and Hamlet was 89%, 91%, 83% and 53%, respectively, with the lowest values for food allergens. There was a good overall correlation of the four techniques, except when the Hamlet method was compared with the other methods for food-specific IgE detection (correlation coefficient < 0.3). These data indicate that CAP, Allercoat and RAST are satisfactory techniques for specific IgE determination, either for inhalants or for food allergens; CAP, however, offers the highest sensitivity without loss of specificity.

Cytoreductive effects of anti-transferrin receptor immunotoxin in a multicellular tumor spheroid model

We have evaluated the sensitivity to immunotoxins (IT) of monolayer and of 200-250 microns multicellular tumor spheroid (MTS) cultures obtained with human breast (MCF7) and glioblastoma (U118) tumor cells and with rat glioblastoma (9L) cells. Monolayer MCF7 and U118 cells were highly sensitive to antitransferrin receptor (anti-TfnR) ricin A chain (RTA)-IT (Tfn-RTA and MAb OKT9-RTA) treatment in the presence of the intracellular RTA-IT enhancing agent human serum albumin-monensin (HSA-Mo) conjugate. A 790- to 2000-fold higher concentration of anti-TfnR IT was instead required to reduce by 50% the volume of individually treated MCF7 spheroids, as evaluated by applying the Gompertz growth model. Monolayer 9L cells showed 230- to 5700-fold lower sensitivity to Tfn-RTA IT than MCF7 and U118 monolayers, yet 9L spheroid cells were almost as sensitive to anti-TfnR IT as monolayer 9L cultures. Binding studies performed with [125I]-Tfn and FITC-labelled anti-TfnR MAb revealed that 9L monolayers and MTS expressed 4.1-fold and 8.8-fold lower amounts of TfnR than MCF7 monolayers and MTS, respectively. However, Tfn bound to TfnR sites of 9L and of MCF7 cells with comparable affinity. Experiments carried out with the diphtheria toxin mutant CRM107 linked to Tfn confirmed the pattern observed with RTA-IT. Monolayers and spheroids showed no considerable differences in sensitivity to ricin toxin. Collectively, these results indicated that the efficacy of IT against 3-D tumors is heavily influenced by the number of target Ag expressed by the tumor cells, as well as by the affinity of IT/toxin-cell interaction.

The complex interplay of the DQB1 and DQA1 loci in the generation of the susceptible and protective phenotype for insulin-dependent diabetes mellitus

IDDM patients of North East Italian region were molecularly typed for their HLA-DQB1 and DQA1 loci by using allele specific oligonucleotide probes and PCR amplified genomic DNA. IDDM status strongly correlated with DQB1 alleles carrying a non-aspartic acid residue in position 57 of DQ beta chain and DQA1 alleles with an arginine residue in position 52 of DQ alpha chain. Genotype analysis revealed that individuals with two DQB1 alleles having a non-aspartic residue in position 57 and two DQA1 alleles with an arginine residue in position 52 had the highest relative risk of disease: they constituted 41% of IDDM patients as compared to 0% of controls. Heterozygosity either at residue 57 of DQB1 or residue 52 of DQA1 was sufficient to abrogate statistical significance for disease association, although 43.6% of IDDM patients were included in these two groups as compared to 21.6% of normal controls. On the other hand the presence of two DQB1 alleles with aspartic acid in position 57 was sufficient to confer resistance to disease irrespective of the DQA1 genotype. Based on the number of possible susceptible heterodimers an individual can form, it was found that 85% of IDDM cases could form two or more heterodimers (two in cis and two in trans), but no IDDM case was found to form one susceptible heterodimer in cis. These results demonstrate that the complete HLA-DQ genotype, more than single DQB1 or DQA1 alleles or DQB1-DQA1 haplotypes, is associated with the highest risk of disease. Screening of the population for preventive purposes and/or early signs of IDDM should then take advantage of this result and "susceptible homozygous" individuals should be followed very closely and considered the first group of choice for possible new therapeutical trials.

Sensitivity of human leukemia cells in exponential or stationary growth phase to anti-CD5 immunotoxins. Role of intracellular processing events

We have assayed the sensitivity of Jurkat cells in different growth phases to an anti-CD5-ricin A chain (ST.1-RTA) immunotoxins (IT). Jurkat cells proliferated exponentially until a stationary growth phase was reached. Proliferating and stationary cells displayed marked differences in sensitivity to ST.1-RTA treatment; the time required to kill one log of target cells (T10) was 70 h in proliferating and 12 h in stationary cells, respectively. Differences in sensitivity to IT treatment were greatly diminished by the addition of the IT enhancer monensin (T10 = 4.9 and 3.5 h in proliferating and stationary cells, respectively). Binding and internalization studies carried out with fluoresceinated ST.1 mAb revealed that the higher sensitivity of stationary cells to ST.1-RTA treatment was not due to an increased uptake or to faster internalization kinetics of IT molecules in this cell population; rather, our data indicated that a different intracellular routing of IT molecules took place in the two cell populations. Mathematical modeling of experimental data allowed us to calculate the efficiency of the intracellular transport of IT molecules toward a subcellular compartment facilitating toxin translocation to the cell cytosol. The IT intracellular processing in stationary cells was 5.5-fold more efficient than in proliferating cells. This value strictly correlated with the higher sensitivity of the stationary cell population to ST.1-RTA treatment.

Sensitivity of human leukemia cells in exponential or stationary growth phase to anti-CD5 immunotoxins. Role of intracellular processing events

We have assayed the sensitivity of Jurkat cells in different growth phases to an anti-CD5-ricin A chain (ST.1-RTA) immunotoxins (IT). Jurkat cells proliferated exponentially until a stationary growth phase was reached. Proliferating and stationary cells displayed marked differences in sensitivity to ST.1-RTA treatment; the time required to kill one log of target cells (T10) was 70 h in proliferating and 12 h in stationary cells, respectively. Differences in sensitivity to IT treatment were greatly diminished by the addition of the IT enhancer monensin (T10 = 4.9 and 3.5 h in proliferating and stationary cells, respectively). Binding and internalization studies carried out with fluoresceinated ST.1 mAb revealed that the higher sensitivity of stationary cells to ST.1-RTA treatment was not due to an increased uptake or to faster internalization kinetics of IT molecules in this cell population; rather, our data indicated that a different intracellular routing of IT molecules took place in the two cell populations. Mathematical modeling of experimental data allowed us to calculate the efficiency of the intracellular transport of IT molecules toward a subcellular compartment facilitating toxin translocation to the cell cytosol. The IT intracellular processing in stationary cells was 5.5-fold more efficient than in proliferating cells. This value strictly correlated with the higher sensitivity of the stationary cell population to ST.1-RTA treatment.

Mechanisms involved in serum-dependent inactivation of the immunotoxin enhancers monensin and carrier-protein-monensin

The immunotoxin-enhancing properties of monensin and of human-serum-albumin-monensin conjugates are severely impaired in the presence of human serum. In this study we have therefore investigated the interaction between serum proteins and monensin leading to the inactivation of monensin function as immunotoxin potentiator. We found that the binding of monensin-specific mAb to thioether-cross-linked or disulfide-cross-linked protein-monensin conjugates is negatively affected by serum, as indicated by immunoenzymic (ELISA) and radioimmunobinding analysis. Size-exclusion chromatography of serum samples indicated that the greatest blocking effect is due to protein components of 40-90 kDa eluting as a broad peak (peak 4). Analysis of the proteins contained within peak 4 by ion-exchange chromatography followed by microsequencing revealed that the major components of peak no. 4 were transferrin, human serum albumin and immunoglobulin fragments. Investigations on the nature of the interactions between serum proteins and monensin leading to monensin inactivation were conducted by affinity chromatography of serum on immobilized human-serum-albumin-monensin conjugates, size-exclusion chromatography, SDS/PAGE analysis of serum-treated human-serum-albumin-monensin conjugates, and evaluation of the stability of immobilized human-serum-albumin-bound 125I-monensin following treatment with serum. Addition of esterase inhibitors (e.g. EDTA, 4-nitrophenyl phosphate) or prior treatment of the serum at 56 degrees C partially reversed the serum effects observed. We conclude that serum proteins block the immunotoxin-enhancing effect of monensin and of human-serum-albumin-monensin conjugates by multiple mechanisms involving hydrophobic and covalent interactions and enzyme-mediated cleavage of protein-bound monensin.

Physiologic target of the Air-1 trans-activator revealed by stable transfection assay

RJ 2.2.5 is a human B cell mutant, derived from Raji cells, which has lost expression of major histocompatibility complex (MHC) class II genes because of a defect in the AIR1 locus function. The MHC class II-positive phenotype can be restored by introducing an active AIR1 locus or its mouse equivalent, Air-1. An example of the latter is the H4 cell hybrid, derived by somatic cell fusion between RJ 2.2.5 and mouse class II-positive spleen cells. H4 contains a single mouse chromosome, autosome 16, in which the Air-1 locus maps, and an entire RJ 2.2.5-derived genome. In the present study we show that the physiologic target of the Air-1 locus product is contained within a limited HLA-DRA promoter sequence of 300 base pairs, encompassing the crucial Y, X, and W cis-acting elements. A plasmid construct, pDRA300neo, containing the HLA-DRA promoter sequence which drives the expression of the neomycin resistance gene, has been stably integrated in the genome of the H4 hybrid. Transfectants selected in the presence of G418 retain mouse chromosome 16 and express the DR genes. On the other hand, transfectants grown in a non-selective medium segregate mouse chromosome 16; this is accompanied by a loss of DRA gene expression and G418 resistance, although pDRA300neo is still integrated in the genome. These results offer scope for using this experimental model to clone the Air-1 gene in a straightforward way.

Susceptibility of human-mouse T cell hybrids to HIV-productive infection

Interspecies human x mouse cell hybrids were used to investigate the genetic basis of human permissivity to HTLV-IIIB infection. T cell hybrids between the mouse BW 51.47 T lymphoma line and normal, PHA-IL-2 activated, human peripheral mononuclear cells (PBMCs) were generated. These hybrids preferentially segregated human chromosomes, as assessed by phenotype and karyotype analysis. Viral integration occurred only in those hybrids expressing CD4+ at the cell surface. However, infectious progeny production was demonstrated only in two of the three CD4+ hybrids tested. By segregation analysis, we could correlate the absence of human chromosomes 1, 3, and 9 with the lack of infectious viral progeny.

Specific humoral response to cows' milk proteins and ovalbumin in children with atopic dermatitis

Serum antibodies to four common food antigens, three cows' milk proteins (casein, alpha-lactalbumin and beta-lactoglobulin) and ovalbumin, were investigated in 21 children with atopic dermatitis (aged 3 months to 3 years) and in 15 age-matched healthy controls. Specific IgE was measured by radioallergosorbent test; an ELISA was developed to detect specific IgG, IgG subclasses and IgA. Specific IgE was found in 76% of patients, while antigen-directed IgG and IgA were present both in patients and healthy controls; IgG to ovalbumin and IgA to alpha-lactalbumin were significantly higher in children with atopic dermatitis. The analysis of the IgG subclass distribution showed different patterns of response, IgG1 and IgG4 being higher in patients (even though statistically significant only for ovalbumin), and IgG2 and IgG3 being lower in this group. The presence of food-specific IgE in the majority of atopic children and the different specific IgG subclass patterns observed in patients and controls may reflect an alteration in the immune response to dietary proteins in atopic dermatitis.

Differential effects of tyrosine kinase inhibition in CD69 antigen expression and lytic activity induced by rIL-2, rIL-12, and rIFN-alpha in human NK cells

The effect of rIL-12 on induction of CD69 antigen expression and cytolytic activity in purified human NK cells was evaluated in comparison to the effects of rIL-2 and rIFN-alpha. It was found that rIL-12 directly induced CD69 antigen expression in NK cells, although the period of incubation required by rIL-12 was longer than the period required by rIL-2 or by rIFN-alpha. Similarly, the cytolytic activity induced by rIL-12 in NK cells against the NK-resistant target cell line Raji was consistently lower than the cytolytic activity induced by rIL-2 or rIFN-alpha when measured after 6 hr of incubation, and increased during the following 18 hr of incubation. To compare the involvement of tyrosin kinases in activation of NK cells induced by rIL-2, rIL-12, and rIFN-alpha, the effect of the specific inhibitor of tyrosin kinases, genistein, was evaluated on induction of CD69 antigen expression and lytic function mediated by the three cytokines. It was found that genistein inhibited CD69 antigen expression induced by rIL-2 and by rIL-12, but not that induced by rIFN-alpha. Unlike the effect on CD69 antigen expression, the cytolytic activity induced by all three cytokines was inhibited by genistein. These results, together with the finding that CD69 antigen expression induced by rIL-2 but not by rIL-12 or rIFN-alpha was inhibited by addition of rIL-4, strongly suggest that IL-2, IL-12, and IFN-alpha mediate their effects, leading to induction of CD69 antigen expression through different activation pathways.

Methodological Analysis of Flowcytometric Measurements of Nuclear Dna in Oncology

The application of specific and proven guidelines is required for flow cytometric nuclear DNA analysis, which can provide precise information on genomic quantitative modifications and the proliferative activity of tumour tissues, clinically useful for prognosis and monitoring of therapy and recurrencies. The authors review the various protocol phases to be followed for a correct analysis and discuss the main problems that can be encountered in the procedural steps and in interpreting the histograms.

HLA-DQB1 typing of north east Italian IDDM patients using amplified DNA, oligonucleotide probes and a rapid DNA-enzyme immunoassay (DEIA)

We report on HLA-DQB1 typing in IDDM patients of north east Italian region using an enzymatic method based on the detection of hybridization reaction between PCR amplified DNA from whole blood and allele specific oligonucleotides by an antibody directed against double stranded DNA (DNA-enzyme immunoassay or DEIA). The method is reliable, simple and sensitive as the classical radioactive method with the advantage of using a universal non radioactive detection reagent. Nineteen families, each including one subject with juvenile insulin-dependent diabetes mellitus (IDDM) were analyzed. A strong association between absence of an aspartic acid (Asp) in position 57 of DQB1 beta chain in homozygous conditions and susceptibility to IDDM was found. In contrast with some previous observations, however, no significant association was found between Asp/non-Asp heterozygous genotype and IDDM. No patients were found with an homozygous Asp/Asp genotype, known to be protective in caucasoid population. Of particular interest was the DQB1 allelic distribution in our population sample. The non-Asp allele most frequently found in IDDM subjects was the DQB1 0201 allele and this finding was statistically significant (Pc value < 0.05, relative risk = 5.01). No significant association was found for any other allele including the DQB1 0302 (Pc value = not significant although with relative risk = 3.28) previously reported as the most frequent allele in IDDM caucasoid patients.

Constitutive expression of CD69 in interspecies T-cell hybrids and locus assignment to human chromosome 12

In this study we describe the generation and characterization of interspecies somatic cell hybrids between human activated mature T cells and mouse BW5147 thymoma cells. A preferential segregation of human chromosomes was observed in the hybrids. Phenotypic analysis of two hybrids and their clones demonstrated coexpression of CD4 and CD69 antigens in the same cells. Segregation analysis of an informative family of hybrids followed by molecular and karyotype studies clearly demonstrated that the locus encoding CD69 antigen mapped to human chromosome 12. Although the expression of CD69 antigen is an early event after T-lymphocyte activation and rapidly declines in absence of exogenous stimuli, in the hybrids described in this study the expression was constitutive, similarly to what was previously found in early thymocyte precursors and mature thymocytes. In this respect it was important to note that the behavior of the hybrids in culture strongly suggested a dominant influence of the thymus-derived mouse tumor cell genome in controlling the constitutive expression of human CD69. These hybrids may thus provide a system to study the genetic and molecular mechanisms controlling the expression and function of this activation antigen.

Different sensitivity to interleukin 4 of interleukin 2- and interferon alpha-induced CD69 antigen expression in human resting NK cells and CD3+, CD4-, CD8- lymphocytes

The effect of rIL-4 on CD69 antigen expression induced by rIL-2 or by rINF-alpha on human resting NK cells and CD3+, CD4-, CD8- T lymphocytes has been investigated. rIL-4 drastically inhibited CD69 antigen expression induced by rIL-2 in both cell types. In contrast, rIL-4 did not alter rINF-alpha-induced CD69 antigen expression. Consistent results were obtained evaluating the cytolytic activity of NK cells against the Raji target cell line: rINF-alpha-induced lytic activity was not inhibited by rIL-4, while rIL-2-induced lytic activity was drastically inhibited. Proliferative activity of NK cells induced by rIL-2, in contrast, was only slightly reduced by rIL-4. rIL-4 did not alter the expression of the beta chain of IL-2 receptor, evaluated in NK cells by indirect immunofluorescence. Expression of the alpha chain of IL-2 receptor could not be detected in NK cells by indirect immunofluorescence. It can therefore be suggested that the selective inhibitory effect of rIL-4 on rIL-2-induced activation of NK cells is not mediated by downregulation of alpha and beta chains of IL-2 receptor.

Blocking effect of human serum but not of cerebrospinal fluid on ricin A chain immunotoxin potentiation by monensin or carrier protein-monensin conjugates

The potentiation of monoclonal antibody/ligand toxin (immunotoxin) cytotoxicity by the ionophore monensin (Mo) or by human serum albumin-monensin (HSA-Mo) conjugates was investigated. Since disulfide cross-linked HSA-Mo (HSA-SPDP-Mo) is rapidly inactivated by human serum (M. Colombatti et al., Cancer Res., 50: 1385-1391, 1990), we synthesized thioether cross-linked HSA-Mo conjugates (HSA-SIA-Mo). HSA-SIA-Mo is resistant to treatment with reducing agents (e.g., glutathione, dithiothreitol) and shows potentiating activity identical to that of Mo or of HSA-SPDP-Mo, enhancing immunotoxin (IT) cytotoxicity 45-35,000-fold. Human leukemic and tumor cell lines are highly sensitive to treatment with IT in combination with Mo, HSA-SPDP-Mo, or HSA-SIA-Mo (concentration required to inhibit protein synthesis by 50%, 10(-10)-2.5 x 10(-13) M). IT potentiation by both types of HSA-Mo conjugates, however, is inhibited by whole human serum. In contrast, human cerebrospinal fluid has no effect on the potentiation of IT by Mo or HSA-Mo conjugates. The serum blocking factors reside mostly in a Mr 40,000-90,000 protein fraction. Serum components of low molecular weight (less than 10,000) show no detectable effect upon the stability of HSA-Mo conjugates. The toxicity of HSA-SIA-Mo in vivo was investigated by intrathecal injections in rats. Concentrations of up to 60 micrograms/kg can be injected into the brain with only transient neurological sequelae. We therefore conclude that if the systemic delivery of HSA-Mo conjugates for the potentiation of ricin A chain-IT presents some limitations due to the blocking effect of serum, the application of HSA-Mo conjugates in combination with ricin A chain-IT for regional tumor therapy in the brain appears more promising.

Immunohistochemical characterization of nurse cells in normal human thymus

Lymphoepithelial complexes known as thymic "nurse" cells (TNC) have been isolated and described in the thymus of several animal species including man. Most of the investigations on TNC have been carried out in enzymatically digested thymuses in which TNC were isolated by differential sedimentation. In the present study we demonstrate TNC in immunohistochemically stained sections of human thymus as ring-shaped cells completely enclosing thymocytes and localized not only in the cortex, but also at the corticomedullary junction where they have not been previously described. TNC expressed epithelial markers [low and high molecular weight keratins identified by 35 beta H11 and 34 beta E12 monoclonal antibodies, a cortical antigen shared with neuroectodermal neoplasms recognized by the GE2 monoclonal antibody, and tissue polypeptide antigen (TPA:B1)], class II histocompatibility antigens (HLA-DR), and thymosin alpha 1. Double staining experiments with the nuclear proliferation-associated antigen Ki-67 and the cortical epithelium marker GE2 showed that most thymocytes enclosed in these cortical TNC were not proliferating. The antigens expressed by TNC indicate that not only cortical, but also medullary epithelial cells are part of the TNC system. The possible role of TNC in the education and maturation of thymocytes is discussed.

A comparative analysis of three different techniques for the detection of breast cancer cells in bone marrow

Three different methods, morphologic, immunocytochemic, and fluorescence activated cell sorter (FC) analysis, were compared with respect to their efficiency in detecting breast cancer cells in bone marrow. In the first series of experiments, the three techniques were compared using bone marrow cells artificially mixed with a known amount of breast cancer cells, whereas in a second series bone marrow from breast cancer patients with bone metastases were used. The following results were obtained: When mixtures of the first series were analyzed, FC analysis detected from 1% to 10% of breast cancer cells in bone marrow (0.2% was a border line value), the morphologic method detected from 0.05% to 10%, and the immunocytochemic method, which was clearly superior, detected breast cancer cells in all mixtures (from 0.00025% to 10%). It was noted that, with both the morphologic and immunocytochemic methods, the percentage of breast cancer cells detected was 2 to 360 times higher than the percentage of added cells, and enrichment was inversely proportional to the percentage of added cells. This result could be a result of different separation of cells during centrifugation due to the different density of breast cancer cells. The superiority of the immunocytochemic method was confirmed in the second series of experiments.

Structural analysis of the CD69 early activation antigen by two monoclonal antibodies directed to different epitopes

The biochemical structure of CD69 early activation antigen has been characterized by means of two newly isolated mAb, namely C1.18 and E16.5. Upon analysis by SDS-PAGE, C1.18-reactive molecules immunoprecipitated from 125I-surface labeled PMA activated PBL consisted of a 32 + 32 kD dimer, a 32 + 26 kD dimer, a 26 + 26 kD dimer and a 21 + 21 kD dimer. E16.5-reactive molecules consisted of a 26 + 26 kD dimer and a 21 + 21 kD dimer. Cross absorption experiments showed that E16.5 mAb reacts with an epitope of the CD69 molecule distinct from the one recognized by C1.18 mAb and present only on a subpopulation of the CD69 molecular pool. The patterns of migration of C1.18- and E16.5-reactive molecules in two-dimensional gel-electrophoresis, under reducing conditions before and after treatment with Endoglycosidase F enzyme suggest that the two mAb recognize the same glycoprotein structure, but in two distinct glycosylation forms, both expressed on the cell surface membrane. Finally, p32, p26 and p21 of CD69 complex obtained from three distinct normal donors did not show appreciable structural polymorphism, by two-dimensional peptide mapping, not only among single subunits within the same individual, but also among homologous subunits in distinct individuals. Further, it was found that CD69 complex is expressed at the cell surface of resting PBL, although at a very reduced level in comparison to PMA activated cells. C1.18 and E16.5 mAb induced comparable cell proliferation and IL-2 production in PBL in the presence of PMA. C1.18 mAb increased intracellular free calcium concn in PMA activated PBL after cross-linking with goat anti mouse Ig, while the effect induced by E16.5 mAb after cross-linking was consistently lower. Finally, it was found that Sepharose-linked C1.18 mAb, in the presence of rIL-2 or PMA, did not induce TNF release from 6 NK cell clones.

Distribution of endocytosed molecules to intracellular acidic environments correlates with immunotoxin activity

We have investigated the internalization to low pH intracellular compartments of transferrin (Tfn), diphtheria toxin (DT) and of anti-cell surface antibodies (MAb) by a cytofluorometric assay based on low pH quenching of fluorescein (FITC) emission. FITC-labelled Tfn, anti-CD3, anti-CD5 and anti-Thy 1.2 MAb internalization resulted in a progressively lower FITC quenching effect. Following internalization, a distinction could be made between molecules that enter low pH compartments without undergoing intracellular degradation (e.g., Tfn, anti-CD3 MAb) and molecules that are internalized through low pH organelles and are then degraded within the cell (e.g., DT). A strict correlation was observed between quenching of internalized FITC-protein fluorescent emission and the cytotoxic activity of DT-based immunotoxins (IT).

Inducible and constitutive MHC class II gene expression. Distinct tissue-specific genetic controls

B cells express MHC class II Ag in a constitutive fashion, whereas macrophages can do so only after induction by a variety of exogenous stimuli. In this study we describe interspecies somatic cell hybrids between the human B cell Raji and the murine macrophage cell P388 D1. This murine cell line does not express detectable levels of class II mRNA. Phenotypic, molecular, and karyotype analysis of a series of hybrids showed that murine macrophage class II genes can be expressed in a constitutive fashion under the control of the human B cell genome. This event is the consequence of de novo accumulation of class II specific mRNA and thus probably reflects activation of transcription. In certain cases the amount of murine class II Ag expressed on the surface of the hybrid cell was significantly higher than the one observed in the parental macrophage cells after induction with IFN-gamma and was not further modified by treatment with the murine lymphokine. Reversion from a murine class II-positive to class II-negative cell surface phenotype in the hybrids correlated with reduced expression of human markers and more important with segregation of human chromosomes. Interestingly, in this case certain hybrids still expressed detectable levels of murine class II mRNA and increased levels of murine invariant chain mRNA when compared with parental P388 D1 murine macrophage cells. These results indicate that constitutive class II gene expression behaves as a dominant trait in B cell x macrophage somatic cell hybrids. Possible mechanisms responsible of the different control of class II gene expression during cell type differentiation are discussed.