An alternatively spliced form of CD79b gene may account for altered B-cell receptor expression in B-chronic lymphocytic leukemia.

Several functional anomalies of B-chronic lymphocytic leukemia (B-CLL) cells may be explained by abnormalities of the B-cell receptor (BCR), a multimeric complex formed by the sIg homodimer and the noncovalently bound heterodimer Igalpha/Igbeta (CD79a/CD79b). Because the expression of the extracellular Ig-like domain of CD79b has been reported to be absent in the cells of most CLL cases, we have investigated the molecular mechanisms that may account for this defect. Peripheral blood lymphocytes (PBL) from 50 patients and two cell lines (MEC1, MEC2) obtained from the PBL of one of them were studied. MEC1, MEC2, and 75% of CLL cases did not express detectable levels of the extracellular Ig-like domain of CD79b, which was nevertheless present in greater than 80% CD19(+) cells from normal donors. In healthy subjects the expression of CD79b was equally distributed in CD5(+) and CD5(-) B-cell subsets. Reverse transcription-polymerase chain reaction (RT-PCR) analysis of CD79b RNA from all patients and from MEC1 and MEC2 cell lines consistently yielded two fragments of different size (709 bp and 397 bp). The 709-bp band corresponds to CD79b entire transcript; the 397-bp band corresponds to an alternatively spliced form lacking exon 3 that encodes the extracellular Ig-like domain. Both fragments were also visible in normal PBL. The expression of the 397-bp fragment was increased in normal activated B cells, while no difference was seen between CD5(+) and CD5(-) B cells. To obtain a more accurate estimate of the relative proportions of the two spliced forms, a radioactive PCR was performed in 13 normal and 22 B-CLL samples and the results analyzed using a digital imager. The mean value of the CD79b to the CD79b internally deleted ratio was 0.64 +/- 0.20 SD in normal donors and 0.44 +/- 0.27 SD in B-CLL (P =.01). Direct sequencing of 397-bp RT-PCR products and of genomic DNA corresponding to exon 3 from MEC1, MEC2, their parental cells, and five fresh B-CLL samples did not show any causal mutation. Single-strand conformation polymorphism analysis of exon 3 performed in 18 additional B-CLL cases showed a single abnormal shift corresponding to a TGT --> TGC polymorphic change at amino acid 122. We propose a role for the alternative splicing of CD79b gene in causing the reduced expression of BCR on the surface of B-CLL cells. As normal B cells also present this variant, the mechanism of CD79b posttranscriptional regulation might reflect the activation stage of the normal B cell from which B-CLL derives.

Loss of phosphoserine polar group asymmetry and inhibition of cholesterol transport in Jurkat cells treated with cholesterylphosphoserine.

Cholesterylphosphoserine (CPHS) is a synthetic ester of cholesterol showing immunosuppressive activity. In the present study, we have used the T cell line Jurkat to investigate its mechanism of action. CPHS incorporates into cells reaching a molar ratio of 0.23 and 3.9 with the total phospholipid and cholesterol content, without inducing necrosis or apoptosis. CPHS incorporation elicits a dose-dependent binding of fluorescein isothiocyanate-labeled annexin V, suggesting that the steroid distributes in the external leaflet of plasma membrane exposing the phosphoserine group to the external cell environment and inserting the steroid ring into the phospholipid bilayer. In agreement with a preferential steroid association with sphingolipids, CPHS is included in a Triton X-100-insoluble complex when mixed with sphingomyelin and cholesterol. CPHS incorporation inhibits the esterification of low density lipoprotein (LDL)-derived cholesterol, producing a minor influence on the endogenous synthesis of cholesterol and on the acyl-CoA:cholesterol acyltransferase activity. In this effect, CPHS is as potent as progesterone (IC50 of 3.5 microM). It is concluded that the insertion of cholesterylphosphoserine (CPHS) in the Jurkat plasma membrane neutralizes the asymmetric distribution of the phosphoserine group and inhibits the movement of cholesterol to the endoplasmic reticulum. As CPHS is a negatively charged steroid, this last effect may be linked to the perturbation of sphingolipid/cholesterol-based microdomains, proposed to play a role in cholesterol trafficking.

Differential effect on TCR:CD3 stimulation of a 90-kD glycoprotein (gp90/Mac-2BP), a member of the scavenger receptor cysteine-rich domain protein family.

We studied the effects of a 90-kD glycoprotein (gp90/Mac-2BP) belonging to the scavenger receptor family, present in normal serum and at increased levels in inflammatory disease and cancer patients, on some T cell function parameters. Whereas the lymphocyte proliferative response to non-specific mitogens such as phytohaemagglutinin (PHA) and concanavalin A (Con A), but not pokeweed mitogen (PWM), was strongly reduced, probably due to the lectin-binding properties of gp90/Mac-2BP, the response to T cell receptor (TCR) agonists such as superantigens and allogeneic cells was potentiated. When lymphocytes were stimulated with different anti-TCR:CD3 MoAbs, both in soluble and solid-phase form, gp90/Mac-2BP was able to down-regulate the proliferative response to anti-CD3 MoAb, whereas the response to anti-TCR alphabeta MoAb was enhanced. A similar differential effect was observed when a MoAb against CD5 (another member of the scavenger receptor superfamily) was added to anti-CD3 or anti-TCR-stimulated cells; anti-CD5 MoAb strongly down-modulated the CD3-mediated response, whereas its presence in culture was associated with potentiation of the response to TCR alphabeta agonists. gp90/Mac-2BP was able per se to up-regulate Ca2+ levels in freshly isolated lymphocytes; moreover, its presence in culture was associated with increased Ca2+ mobilization following stimulation with anti-TCR alphabeta, but not anti-CD3 MoAb. These data indicate that gp90/Mac-2BP could be able to influence some immune responses, possibly through multiple homologous interactions with other members of the scavenger receptor family; moreover, our findings suggest that signalling through the different components of the TCR:CD3 complex may follow distinct activation pathways into the cells.

Pseudotyping of Moloney leukemia virus-based retroviral vectors with simian immunodeficiency virus envelope leads to targeted infection of human CD4+ lymphoid cells.

In view of our recent findings that a truncated form of the envelope (Env) glycoprotein of human immunodeficiency virus type 1 (HIV-1) was efficiently incorporated into MoMLV particles, we studied the generation of Moloney murine leukemia virus (MoMLV)/simian immunodeficiency virus (SIV) pseudotypes. Unlike HIV-1, both the wild-type SIV Env and a truncated form, which lacks most of the cytoplasmic domain of the transmembrane glycoprotein, were incorporated into MoMLV particles and generated infectious retroviral vectors which could transduce CD4+ sMAGI macaque cells. The infection depended on target cell CD4 expression, and was neutralized by both soluble CD4 and sera from SIV-infected macaques. We also observed pseudotype-mediated gene transfer of a green fluorescent protein marker into the CD4+ CEMX174 and C8166 lymphoid cell lines. More importantly, primary human lymphocytes were also successfully transduced ex vivo by MoMLV/SIV pseudotypes, albeit at lower efficiency, and gene transfer was specifically restricted to the CD4+ subset. These findings demonstrate that MoMLV/SIV pseudotypes can be used to transduce cells which are susceptible to SIV infection, and thus might be advantageously employed in animal models for direct in vivo delivery of gene therapy-based approaches.

Therapeutic effect of interleukin 12 on mouse haemangiosarcomas is not associated with an increased anti-tumour cytotoxic T-lymphocyte activity.

In syngeneic mice, the H5V polyoma middle-T oncogene-transformed endothelioma cell line induces Kaposi's sarcoma-like cavernous haemangiomas that regress transiently, probably because of an anti-tumour immune response, but eventually grow progressively and kill the host. To evaluate the generation of tumour-specific cytotoxic T lymphocytes (CTLs), spleen cells of tumour-bearing mice were restimulated with irradiated H5V cells in mixed leucocyte-tumour cell cultures. Tumour-specific CTLs were demonstrable only when low numbers of H5V stimulator cells were used (<1 H5V cell per 50 splenocytes). We found that H5V cells secrete immunosuppressive mediators because CTL generation was blocked when H5V cells culture supernatants were added to allogeneic mixed leucocyte cultures. As numerous tumour-derived immunosuppressive mediators may interfere with interleukin 12 (IL-12) production, we tested whether IL-12 treatment of the tumour-bearing mice would augment their immune response and thus suppress tumour growth. Indeed, IL-12 inhibited tumour growth and prevented mortality, but did not increase anti-H5V CTL generation either in vitro or in vivo. Moreover, the anti-tumour activity in IL-12-treated mice was abrogated by anti-interferon (IFN)-gamma monoclonal antibody (MAb) co-administration. These results strongly suggest that the anti-tumour effect of IL-12 is principally mediated by IFN-gamma release that in turn blocks H5V cell proliferation and induces the release of factors that suppress angiogenesis.

Lymphoproliferative disease in human peripheral blood mononuclear cell-injected SCID mice. IV. Differential activation of human Th1 and Th2 lymphocytes and influence of the atopic status on lymphoma development.

Intraperitoneal transfer of PBMC from EBV+ donors into SCID mice leads to high human Ig levels in mouse serum and B cell lymphoproliferative disease. As these events depend on the activation of coinjected human T cells, we addressed the behavior of the Th1 and Th2 subsets in this model. Production of IFN-gamma, but not of Th2 cytokines such as IL-4, was detected in culture supernatants of PBMC stimulated in vitro with mouse splenocytes. Moreover, anti-CD3 stimulation of the human cells recovered from mice brought about IFN-gamma, but not IL-4, synthesis; on the other hand, PCR and in situ hybridization analysis of ex vivo-recovered cells disclosed the presence of mRNA for both cytokines following in vitro restimulation, thus suggesting posttranscriptional regulation of IL-4 gene expression. When SCID mice were inoculated with PBMC from atopic donors, whose Th1/Th2 profile displays an imbalance toward Th2 cells, tumor development rates were lower, and tumor latency was higher, compared with those in mice injected with PBMC from normal donors. Isotypic analysis of human Ig in mouse serum showed the exclusive presence of IFN-gamma-driven IgG subclasses; in addition, human IgE were low or undetectable in most cases. These findings indicate that following transfer into SCID mice, human Th1 lymphocytes undergo preferential activation, whereas Th2 function is down-regulated. Th1 lymphocytes probably are a major component in promoting EBV+ B cell expansion and tumor development; the individual Th1/Th2 profile could in part account for the as yet unexplained donor variability in tumor generation in this experimental model.

Cell cycle-dependent metabolism of pyrimidine deoxynucleoside triphosphates in CEM cells.

We incorporated 3H-labeled thymidine, deoxycytidine, or cytidine into dNTPs and DNA of exponentially growing CEM cells. G1 and S phase cells were separated by centrifugal elutriation, and the size and specific activity of dNTP pools were determined to study the cell cycle-dependent regulation of specific dNTP synthesizing enzymes in their metabolic context. With [3H]thymidine, we confirm the earlier demonstrated S phase specificity of thymidine kinase. Incorporation of radioactivity from [5-3H]deoxycytidine into dCTP occurred almost exclusively in G1 cells. During S phase, de novo synthesis by ribonucleotide reductase was switched on, resulting in a 70-fold dilution of [3H]dCTP, confirming that ribonucleotide reductase is an S phase-specific enzyme, whereas deoxycytidine kinase is not. [5-3H]Cytidine appeared in dCTP almost to the same extent in G1 as in S phase, despite the S phase specificity of ribonucleotide reductase. During S phase, DNA replication greatly increased the turnover of dCTP, requiring a corresponding increase in ribonucleotide reductase activity. During G1, the enzyme maintained activity to provide dNTPs for DNA repair and mitochondrial DNA synthesis. The poor incorporation of isotope from deoxycytidine into DNA earlier led to the suggestion that the nucleoside is used only for DNA repair (Xu, Y-Z., Peng, H., and Plunkett, W. (1995) J. Biol. Chem. 270, 631-637). The poor phosphorylation of deoxycytidine in S phase provides a better explanation.

c-erbB-2/neu protein expression, DNA ploidy and S phase in breast cancer.

DNA content and c-erbB2/neu protein (p185) expression were evaluated by flow-cytometry and ELISA, respectively, in 166 specimens of primary breast cancer. A non-diploid DNA content was found in 88 tumours (53%), with the DNA index ranging from 0.7-2.7. S phase fraction (SPF) evaluation, performed in 130 cases, showed significantly higher values in aneuploid than in diploid tumours (median values, 17.3% and 5.8%, respectively). Thirty-six tumours (21.6%) showed p185 overexpression, while 45 (27.1%) and 85 (51.3%) showed intermediate and low expression, respectively. A good correlation (P = 0.0023) was found between DNA content and p185 positivity. Tumours with high p185 values were mainly aneuploid, while tumours with intermediate or low expression had variable degrees of DNA content. Furthermore, p185 concentration was significantly higher in aneuploid than in diploid tumours (P = 0.009). The highest rate of p185 (+) cases and the highest p185 concentrations occurred in triploid (1.3 < D.I. < or = 1.7), compared to the other tumours. SPF values and p185 positivity rates were not significantly correlated in diploid and in aneuploid tumours.

Synergistic antiviral action of ribonucleotide reductase inhibitors and 3'-azido-3'-deoxythymidine on HIV type 1 infection in vitro.

Ribonucleotide reductase inhibitors reduce the cellular supply of DNA precursors(dNTP) by interfering with their de novo synthesis. A secondary effect is the stimulation of the uptake and phosphorylation of extracellular deoxynucleosides, including their analogs, e.g., 3'-azidothymidine (AZT). Both effects are relevant to HIV replication, which requires dNTP and is impaired by the triphosphate of AZT. Earlier we demonstrated that ribonucleotide reductase inhibitors--hydroxyurea, and two deoxycytidine analogs specifically active in lymphoid cells--increased the phosphorylation of AZT in CEM cells by prolonging the S phase of the cell cycle. Here we tested the effects of long-term treatments on HIV proliferation in CEM cells and stimulated human lymphocytes infected with HIV-1IIIB. Treatment with low doses of AZT (0.05-0.1 microM) and either hydroxyurea (25-100 microM) or 2'-azido-2'-deoxycytidine (0.25-4 microM) lasted 2 weeks, during which p24 in the culture medium was monitored. Noninfected CEM cells were treated in parallel to measure the inhibition of cell growth, distribution along the cell cycle, dNTP pool size, and level of tritiated AZT phosphorylation. A clear synergism between AZT and ribonucleotide reductase inhibitors was observed at nontoxic doses that induced only minor changes in the cellular parameters measured. The reductase inhibitors by themselves interfered with replication only at doses that inhibited cell proliferation.

A CD3+CD8+ T cell population lacking CD5 antigen expression is expanded in peripheral blood of human immunodeficiency virus-infected patients.

In this study we analyzed the behavior of a CD3+ T cell subpopulation lacking CD5 antigen expression in PBMC from HIV-1-infected patients. CD3+CD5- lymphocytes were greatly increased in peripheral blood of HIV-1+ patients, accounting for 20.6 +/- 9.9% of the total CD3+ cells, compared to seronegative individuals (5.5 +/- 3.2%). In both seropositive patients and controls, CD3+CD5- cells belonged to the CD8+ compartment; they were nonactivated, TCR alpha/beta+, naive lymphocytes, and in seronegative individuals preferentially expressed NK cell-associated markers, such as CD11b, CD16, CD56, and CD57. The phenotypic profile of this subset was slightly different in seropositive patients; while TCR expression and CD45RA/RO profile were comparable, CD11b and CD16 expression was lower compared to control figures, while CD56 expression was not changed, and CD57 expression was enhanced. Functional analysis of enriched CD3+CD8+CD5- cells showed an impaired ability to proliferate in response to mitogenic and antigenic stimuli; despite their NK-like phenotype, CD3+CD8+CD5- cells did not exert any NK cytotoxic activity, and only a lectin-dependent cytotoxic potential could be evidenced in this population. These results describe a novel alteration in the lymphocytes phenotypic profile during HIV-1 infection, involving a "transitional" population, which shares some properties of the T and of the NK cell lineage.

Role of anti-LFA-1 and anti-ICAM-1 combined MAb treatment in the rejection of tumors induced by Moloney murine sarcoma virus (M-MSV).

We investigated the effect of combined treatment with anti-LFA-1 and anti-ICAM-1 monoclonal antibodies (MAbs) in the immune reaction to Moloney-murine-sarcoma-virus(M-MSV)-induced tumors, which spontaneously regress due to the generation of a strong virus-specific cytotoxic-T-lymphocyte(CTL) response. Repeated systemic administration of both MAbs to M-MSV-injected mice enhanced tumor growth and delayed regression, while treatment with a single MAb had a similar, though less pronounced, effect. The immune depression achieved could not be attributed to lymphocyte depletion, because no reduction in the total number of leukocytes was detected in the peripheral blood or spleen of these mice. However, anti-LFA-I MAb, alone or in combination with anti-ICAM-I MAb, prevented lymphocyte homing in tumor-draining lymph nodes. Cytofluorimetric analysis disclosed a profound down-modulation of LFA-I and ICAM-I molecule expression on T cells following in vivo MAb treatment. Moreover, in anti-LFA-I MAb-treated mice, the receptor was coated to saturation, while anti-ICAM-I MAb treatment brought about ICAM-I-molecule-coating levels below saturation. Evaluation of M-MSV-specific CTL precursor (p) frequency in lymphoid organs of mice receiving combined MAb treatment showed that CTL generation was greatly reduced 10 days after M-MSV injection, and returned to control levels by day 15. Our findings indicate that systemic administration of MAbs to LFA-I and ICAM-I molecules brings about a strong immune suppressive effect which is mainly due to a block in T-lymphocyte re-circulation, and activation by tumor cells. However, this immune-depressive effect is only temporary, and strictly dependent on continuous MAb administration. Thus, our data suggest that treatment with anti-LFA-I and anti-ICAM-I MAbs combined is unable to induce T-cell tolerance in a highly immunogenic system.

Minimal sequence requirements for synthetic peptides derived from the V3 loop of the human immunodeficiency virus type 1 (HIV-1) to enhance HIV-1 binding to cells and infection.

We previously demonstrated that a 23-mer peptide (DB3) derived from the V3 loop of the surface glycoprotein of HIV-1 MN strain was able to bind to soluble CD4 and enhance HIV-1 infection. The mechanism and structural features required for these biological activities were studied by using shortened DB3 derivatives and DB3 analogs carrying single amino acid substitutions. We found that peptides in which the aromatic amino acid in position 15 or 16 had been replaced by an uncharged hydrophobic residue (DB3-I15 and DB3-I16), analogs in which positively charged amino acids were replaced by corresponding D-enantiomers, and shortened DB3-derivatives lost both enhancing activity and ability to bind to soluble CD4. Other peptide variants in which a positively charged amino acid was replaced by asparagine at positions 3 (DB3-N3), 6 (DB3-N6), and 19 (DB3-N19), respectively, retained both enhancing and binding activities, although with different efficiencies. The CD4 binder peptides DB3 and DB3-N19, but none of the CD4 nonbinder peptides, enhanced CD4 expression on peptide-treated cells as well as gp120 binding to both CD4+ cells and soluble CD4. These findings strongly suggest that the peptide/CD4 interaction induced an increase in both CD4 expression and CD4/gp120 binding affinity, which in turn mediated the enhancement of viral infection. A model of the structural conformation of DB3 peptide required for its biological activities is discussed.

HIV-1 infection of the thymus: evidence for a cytopathic and thymotropic viral variant in vivo.

As thymocyte infection may represent one of the mechanisms responsible for CD4+ T lymphocyte depletion in HIV-1-infected individuals, we studied the occurrence of HIV-1 infection in the thymus in vivo. Thymus (THYPD) and peripheral blood (PBLPD) primary viral isolates were obtained from an HIV-1-infected patient; restriction pattern analysis revealed the presence of a viral variant (THY) in the thymus isolate, from which biological viral clones containing this variant were obtained by limiting dilution infection of Molt-3 cells. The biological phenotype of the viral isolates and THY clones was studied in different cell lines and primary cultures. PBLPD, THYPD, and THY clones could efficiently infect T cell lines; the thymic variant showed a higher cytopathic activity in T cell lines, and a higher replication capacity in both unfractionated and CD4+CD8(+)-enriched primary thymocytes. Sequence analysis of the viral population patterns in vivo confirmed the presence of the THY variant in the thymic compartment, and revealed that the degree of V3 loop heterogeneity was higher in the thymocytes of the patient than in the peripheral blood lymphocytes. In addition to confirming thymocyte infection in vivo, our data also indicate that a differential distribution of viral variants may occur among different body compartments in a single individual; the emergence of cytopathic and tissue-specific variants in the thymus may play a relevant role in the pathogenesis of HIV-1 disease.

Inhibition of protein tyrosine phosphorylation prevents T-cell-mediated cytotoxicity.

Several lines of evidence point to a central role for protein tyrosine kinases (PTKs) in the signal transduction cascade initiated by T-cell receptor (TCR) engagement. In cytotoxic T lymphocytes (CTL), TCR crosslinking leads to activation of the lytic process which includes conjugate formation, lethal hit delivery, and events leading to target cell death. We studied the role of PTKs in antigen-specific cytotoxicity exerted by both in vivo activated and in vitro maintained CTL. We found that the PTK inhibitors herbimycin A and genistein blocked T-cell-mediated lysis in a dose-dependent manner. Lack of cytotoxic function was not due to abrogation of conjugate formation, but was associated with inhibition of both granule exocytosis and phosphatidylinositides turnover, thus indicating that PTK activity is an obligatory event for the activation of antigen-specific CTL effector function.

Inhibition of ribonucleotide reductase by 2'-substituted deoxycytidine analogs: possible application in AIDS treatment.

After phosphorylation to the corresponding diphosphates, 2'-azido-2'-deoxycytidine and 2'-difluorocytidine act as powerful inhibitors of ribonucleotide reductase. Phosphorylation requires deoxycytidine kinase, an enzyme with particularly high activity in lymphoid cells. Therefore, the deoxycytidine analogs can be expected to inhibit the reductase with some specificity for the lymphoid system. Pretreatment of human CEM lymphoblasts with the analogs considerably increased the phosphorylation of 3'-deoxy-3'-azidothymidine (AzT). The increased phosphorylation of AzT is caused by a prolongation of the S phase of the cell cycle. Our results suggest the possibility of a combination of 2'-substituted deoxycytidine analogs with AzT in the treatment of AIDS. Gao et al. [Gao, W.-Y., Cara, A., Gallo, R. C. & Lori, F. (1993) Proc. Natl. Acad. Sci. USA 90, 8925-8928] have suggested the use of the ribonucleotide reductase inhibitor hydroxyurea for this purpose, since the resulting decrease in the size of deoxyribonucleotide pools decreases the processivity of the HIV reverse transcriptase. From our results it would appear that the 2'-substituted deoxycytidine analogs might be preferable to hydroxyurea.

HIV-1 induces down-regulation of bcl-2 expression and death by apoptosis of EBV-immortalized B cells: a model for a persistent "self-limiting" HIV-1 infection.

Interactions between HIV-1 and EBV were studied in HIV-1-infected EBV-positive lymphoblastoid B cells. Following in vitro exposure of B cells to HIV-1, the number of infected cells reached a plateau (25-35%) in approximately 20 days and remained fairly stable thereafter, despite the presence of infectious virus in culture supernatants. HIV-1-positive (gp120+) were separated from HIV-1-negative (gp120-) cells, and the two fractions were further characterized for EBV antigens, bcl-2 expression, and growth capacity in vitro. Compared to gp120- cells, EBNA 1, EBNA 2, and LMP 1 were down-regulated, and the episomal form of EBV-DNA was dramatically decreased in the gp120+ cells. When plated in culture gp120+, but not gp120-, cells died; BZLF1 antigen was not expressed, thus ruling out a reactivation of the EBV lytic cycle. Cytofluorometric, morphological, and molecular analyses disclosed that gp120+ cell death was due instead to apoptosis; evidence of bcl-2 down-regulation in these cells was consistent with this finding. gp120+ cell apoptosis contributed to keeping the level of HIV-1-infected cells at a steady state in the unfractionated culture, where persistent infection was maintained by HIV-1 transmission to B cells newly arising from the proliferation of HIV-1-uninfected cells.

Mechanism of action of the monosialoganglioside GM1 as a modulator of CD4 expression. Evidence that GM1-CD4 interaction triggers dissociation of p56lck from CD4, and CD4 internalization and degradation.

Analyzing the mechanisms underlying the capability of the monosialoganglioside GM1 to induce CD4 modulation we observed that GM1 has a dual effect on the CD4 molecule. GM1 treatment of the lymphoma cell line MOLT-3 and CD4-transfected HeLa cells for times shorter than 30 min prevented binding of monoclonal antibodies (mAbs) recognizing epitopes located within the first NH2-terminal domains of CD4, but not of the OKT4 mAb, which binds to the region of CD4 proximal to the transmembrane domain. However, no binding of the OKT4 mAb was observed after a few hours of treatment with GM1 in both MOLT-3 cells and HeLa cells transfected with an intact CD4 molecule, but not in HeLa cells transfected with a CD4 molecule lacking the bulk of the cytoplasmic domain, suggesting that modulation of CD4 by GM1 depends on the integrity of the cytoplasmic domain. GM1 treatment blocked binding of several mAbs which recognize epitopes located within the first two NH2-terminal domains of CD4 and did not induce CD4 down-modulation if MOLT-3 cells were preincubated with the OKT4A or the OKT4 mAbs. Immunoprecipitation studies with [35S]methionine-labeled MOLT-3 cells showed that GM1-induced CD4 down-modulation was accompanied by CD4 degradation, and this was preceded by dissociation of p56lck from CD4. GM1-induced CD4 down-modulation, dissociation of p56lck from CD4, and CD4 degradation were unaffected by staurosporine, which, on the contrary, blocked these events in response to phorbol 12-myristate 13-acetate. These observations demonstrate that the first action of GM1 is to mask epitopes located within the first two NH2-terminal domains; then, GM1 triggers protein kinase C-independent signals which cause p56lck dissociation from CD4 and the delivery of the molecule to an intracellular compartment where it is eventually degraded.

Role of adhesion molecules in the immune reaction to M-MSV-induced tumors.

We have investigated the in vivo role of 2 different adhesion molecules, LFA-1 and LECAM-1, in the immune reaction to Moloney-murine-sarcoma-virus(M-MSV)-induced tumors, which undergo a peculiar spontaneous regression due to generation of a strong virus-specific cytotoxic-T-lymphocyte(CTL) response. Repeated administration of anti-LFA-1 monoclonal antibody (FD441.8 MAb), i.p. or at the site of virus inoculation, enhanced tumor growth and delayed regression, while i.p. administration of anti-LECAM-1 MEL-14 MAb gave rise to tumors that grew progressively and caused host death. Evaluation of the immunological response in MAb-treated mice showed reduced generation of virus-specific CTL precursors (p) in the spleen of animals given FD441.8 MAb i.p.; CTLp frequency in locally treated mice overlapped with that of control mice injected with virus only. FD441.8 MAb treatment did not interfere with CTL homing in the tumor, since the frequency of M-MSV-specific CTLps in sarcomas was similar in treated and control mice. Cytofluorimetric analysis indicated that the majority of tumor-infiltrating lymphocytes (TIL) from MAb-treated mice were covered by anti-LFA-1 MAb, and lacked cytotoxic activity when assayed against target cells bearing relevant tumor antigens. Instead, in mice injected i.p. with MEL-14 MAb, a very low frequency of CTLps was detected in lymph nodes draining the tumor area, and within the tumor. Our results indicate that enhanced tumor growth, depending on the MAb used, is the resultant of an inhibitory effect on different T-lymphocyte functions. Tumor progression in anti-LFA-1 MAb-injected mice is explained mostly by blockage of CTL lytic activity at the tumor site; in mice receiving i.p. MEL-14 MAb treatment, by the failure of naive T lymphocytes to enter peripheral lymph nodes and subsequently by the lack of generation of tumor-specific CTLs.

Lymphoma induction by human B cells in scid mice.

Lymphoma development was studied in scid mice injected i.p. with PBMC from EBV-positive donors. Most injected mice developed oligo/monoclonal B-cell tumors within 4 months after the inoculation; EBV genome was found in tumor cells. Removal of T lymphocytes from the injected cell populations prevented lymphoma development in all mice, suggesting that T-cell-derived factors are involved in the expansion of the latently EBV-infected B-cell population within the immunodeficient host.