Secreted virus-encoded proteins reflect murine cytomegalovirus productivity in organs

Mouse infection with murine cytomegalovirus (MCMV) is an established model for studying human cytomegalovirus infection. In this study, the relationship was analyzed between MCMV activity in organs of infected mice and the presence of infectious virus (viremia), viral genomes (DNAemia), or secreted virus-encoded proteins in the blood. For the latter, 2 recombinant viruses were constructed that encode for the hepatitis B virus surface antigen and the secreted alkaline phosphatase, respectively, as secreted marker proteins. The secreted markers correlated better with the infection in organs than DNAemia and viremia. The marker protein assays can serve as practical and sensitive tools for longitudinal monitoring of MCMV infection in individual mice.

Immune responses and cytokine induction in the development of severe hepatitis during acute infections with murine cytomegalovirus

Salivary gland-derived murine cytomegalovirus (SGV) infections of mice have been widely used as models of human cytomegalovirus infections and in the study of CMV biology. Still, many aspects of SGV pathogenesis are not clearly defined. Fatal and non-fatal SGV infections were investigated to characterize pathogenetic correlates of mortality and to assess the role of the immune response in disease progression. Suppression of immune responses was observed in both lethal and sublethal infections. Depletion of immune cell populations in spleen, however, correlated with severe CMV-induced hepatitis and mortality. In addition, T cell depletion studies indicated a requirement for this immune cell subset in control of liver damage and survival of infected mice. Examination of cytokine responses revealed a previously undescribed shock-like syndrome in lethally-infected mice characterized by high levels of tumor necrosis factor alpha and interferon gamma. Furthermore, the sites of tumor necrosis factor alpha gene induction did not strictly correlate with either viral load or the sites of tissue damage during infection. Taken together, these findings define the pathogenetic progression of disease as it relates to disease outcome and suggests that organ-specific differences in cytokine induction play a significant role in the late stages of acute lethal MCMV infections.

Ganglioside expression in tissues of mice lacking the tumor necrosis factor receptor 1

This study presents a comparative analysis of gangliosides from lymphoid (spleen and thymus) and other tissues (brain, liver, lung, muscle) of C57BL/6 mice homozygous (-/-) and heterozygous (+/-) for the tumor necrosis factor receptor 1 (TNFRp55). Quantitative and qualitative differences in the expression of the lipid-bound N-acetylneuraminic (Neu5Ac) and N-glycolylneuraminic acid (Neu5Gc) and of various ganglioside biosynthesis pathways were detected between the tissues of the TNFRp55 -/- and the control TNFRp55 +/- mice. Sialic acid profiles showed a strong decrease in the absolute amount of sialic acids (Neu5Ac + Neu5Gc) in the lungs and thymus of homozygous (1.41 and 0.3 ng/mg wet weight, respectively) compared with control heterozygous animals (7.18 and 2.05 ng/mg wet weight, respectively). Considerable differences of Neu5Ac/Neu5Gc ratios in the lungs, muscle, spleen, and thymus were also detected. The gangliosides GM3(Neu5Ac) and GM3(Neu5Gc) were the dominant gangliosides in the lungs of the control animals, whereas the knockout mice almost completely lacked these structures in this organ. Reduced expression of GM1b-type gangliosides (GM1b and GalNAc-GM1b) was also found in the lungs, spleen, and thymus of the TNFRp55 knockout mice. On the other hand, neolacto-series gangliosides were more abundant in the lungs, brain, and muscle of the knockout mice, whereas their expression in the liver, spleen, and thymus was similar in both groups of animals. This study provides in vivo evidence that TNF signaling via the TNFRp55 is involved in the acquisition of a distinct ganglioside assembly in different mouse organs. TNFRp55 signaling seems to be especially important for the activation of the GM1b-type ganglioside biosynthetic pathway that is a unique characteristic of the mouse lymphoid tissues.

Hierarchical and redundant lymphocyte subset control precludes cytomegalovirus replication during latent infection

Reactivation from latent cytomegalovirus (CMV) infection is often associated with conditions of immunosuppression and can result in fatal disease. Whether the maintenance of systemic CMV latency is mainly governed by factors of the infected cell or by immune control functions is unknown. Likewise, the putative immune control mechanisms which could prevent the induction and spread of recurrent CMV infection are not clearly identified. We took advantage of latently infected B cell-deficient mice and a sensitive method for virus detection to study CMV reactivation after ablation of lymphocyte subsets. A crucial role of both T lymphocytes and natural killer (NK) cells was demonstrated. Within 5 d after depletion of lymphocytes, productive infection occurred in 50% of mice, and 14 d later 100% of mice exhibited recurrent infection. A hierarchy of immune control functions of CD8(+), NK, and CD4(+) cells was established. Reactivation was rare if only one of the lymphocyte subsets was depleted, but was evident after removal of a further subset, indicating a functional redundancy of control mechanisms. The salivary glands were identified as the site of most rapid virus shedding, followed by the detection of recurrent virus in the lungs, and eventually in the spleen. Our findings document a previously unknown propensity of latent CMV genomes to enter productive infection immediately and with a high frequency after immune cell depletion. The data indicate that only the sustained cellular immune control prevents CMV replication and restricts the viral genome to a systemic state of latency.

Virus attenuation after deletion of the cytomegalovirus Fc receptor gene is not due to antibody control

The murine cytomegalovirus (MCMV) fcr-1 gene codes for a glycoprotein located at the surface of infected cells which strongly binds the Fc fragment of murine immunoglobulin G. To determine the biological significance of the fcr-1 gene during viral infection, we constructed MCMV fcr-1 deletion mutants and revertants. The fcr-1 gene was disrupted by insertion of the Escherichia coli lacZ gene. In another mutant, the marker gene was also deleted, by recombinase cre. As expected for its hypothetical role in immunoevasion, the infection of mice with fcr-1 deletion mutants resulted in significantly restricted replication in comparison with wild-type MCMV and revertant virus. In mutant mice lacking antibodies, however, the fcr-1 deletion mutants also replicated poorly. This demonstrated that the cell surface-expressed viral glycoprotein with FcR activity strongly modulates the virus-host interaction but that this biological function is not caused by the immunoglobulin binding property.

Lack of MHC class I complex expression has no effect on spread and control of cytomegalovirus infection in vivo

It has been claimed that MHC class I proteins serve as receptors for murine cytomegalovirus (MCMV) and that this interaction is the most important mechanism for virus entry in most cells. This claim is based on the observation that the MHC haplotype contributes to the susceptibility to cytomegalovirus (CMV) infection in vivo. Results from in vitro studies support the concept that stable expression of correctly folded MHC class I molecules contributes to infection, since the individual properties of MHC class I alleles, the availability of beta 2-microglobulin (beta 2m) and also the degree of peptide charging of the MHC class I heavy chain beta 2m heterodimers determined the infection phenotype of cell lines. To assess the biological relevance of proper MHC class I expression we investigated CMV infection in beta 2m-deficient mice which fail to express ternary MHC class I complexes and lack peripheral CD8+ T lymphocytes. We found that organ virus titres and virus clearance kinetics were not altered in beta 2m mutant mice. In addition, there was no indication of diminished virus propagation in beta 2m-/- embryonic fibroblasts. beta 2m-/- mice suffered from the lack of CD8+ T lymphocytes that was partially compensated for by the function of CD4+ T lymphocytes. An organ-specific anti-virus function of natural killer (NK) cells was observed, independent from the beta 2m deletion. The immune control unique for salivary gland infection was maintained. From the data presented here, we confirm the role of MHC class I molecules in the immune surveillance of CMV infection but question the biological impact of correct MHC class I complexes for productive infection.

Bone turnover in homozygous beta 2-microglobulin knock-out mice does not differ from that of their heterozygous littermates

beta 2-Microglobulin is a constituent of the class I major histocompatibility complex (MHC) molecule and crucial for its normal function in cell recognition. It has also been isolated from bone and shown to regulate bone metabolism and to be altered in various bone diseases. In order to further investigate the role of the immune system in bone metabolism, we studied basic properties of bone physiology in beta 2-microglobulin-deficient mice created by the technique of gene knock-out. Ten week-old male offspring homozygous (non-functional class I MHC molecule) or heterozygous (functional class I MHC molecule) for beta 2-microglobulin knock-out gene did not differ in the following measures of bone turnover: femur length, dry and ash weight and calcium content, serum calcium concentration and alkaline phosphatase activity, total vertebral tissue area, trabecular bone volume, osteid surface, osteoclast surface and mineral apposition rate. These data indicate that the bone turnover in beta 2-microglobulin-deficient mice is appropriate for the stage of their skeletal maturation.

Antibodies are not essential for the resolution of primary cytomegalovirus infection but limit dissemination of recurrent virus

Virus shedding from the epithelial cells of the serous acini of salivary glands is a major source for the horizontal transmission of cytomegalovirus. These cells are, different to other tissues, exempt from CD8 T lymphocyte control. CD4 T lymphocytes are essential to terminate the productive infection. Here, we prove that T-B cooperation and the production of antibodies are not required for this process. For the infection with murine cytomegalovirus, mutant mice were used which do not produce antibodies because of a disrupted membrane exon of the immunoglobulin mu chain gene. Also, in these mice the virus clearance from salivary glands is a function of CD4 T lymphocytes. However, these mice clear the virus and establish viral latency with a kinetics that is distinguishable from normal mice. Reactivation from virus latency is the only stage at which the absence of antibodies alters the phenotype of infection. In immunoglobulin-deficient mice, virus recurrence results in higher virus titers. The adoptive serum transfer proved that antibody is the limited factor that prevents virus dissemination in the immunodeficient host.

Restoration of cytomegalovirus antigen presentation by gamma interferon combats viral escape

An immediate-early protein of murine cytomegalovirus (MCMV), pp89, elicits an immunodominant and protective major histocompatibility complex (MHC) class I Ld-restricted CD8+ T-lymphocyte response. Remarkably, presentation of the naturally processed peptide of pp89, the nonapeptide YPHFMPTNL, is abolished during permissive MCMV infection in vitro. This defect in pp89 presentation is due to the expression of MCMV early gene functions that specifically block the transport of peptide-charged MHC class I complexes to the cell surface (M. Del Val, H. Hengel, H. Häcker, U. Hartlaub, T. Ruppert, P. Lucin, and U. H. Koszinowski, J. Exp. Med. 176:729-738, 1992). Here, we demonstrate that MCMV-specific CD8+ T lymphocytes can reconstitute pp89 presentation in a parakrine fashion. The lymphocytes mediate the restoration of antigen presentation by MCMV-infected cells by releasing gamma interferon (IFN-gamma). IFN-gamma has no effect on synthesis and stability of the viral antigen pp89 nor does it interfere with the expression of viral early genes and their inhibitory effect on MHC class I molecular maturation. IFN-gamma results in a 25-fold increase in the synthesis of MHC class I molecules and a similar increase in the abundance of pp89-derived peptide. Many of the MHC molecules remain retained by the viral effect, but a surplus of MHC molecules escapes the effect and provides the effective surface presentation of the peptide. Adoptive cell transfer studies demonstrate the IFN-gamma dependence of CD8+ T-lymphocyte function in vivo. Altogether, these data reconcile the paradoxical findings of an impaired pp89 presentation in vitro in parallel with pp89-specific CD8+ T-cell protection in vivo. The results also imply a role of IFN-gamma in the T-lymphocyte-mediated control of cytomegalovirus infection. The known propensity of cytomegalovirus to cause serious disease in the immunocompromised host is discussed in the light of these findings.

The conditions of primary infection define the load of latent viral genome in organs and the risk of recurrent cytomegalovirus disease

Recurrence of cytomegalovirus (CMV) from latency is a frequent cause of disease in immunocompromised patients. To date, there is no explanation for the diversity in the clinical manifestations. Primary infection can occur perinatally or later in life, and inevitably results in latent infection. Seropositivity for antibodies against CMV is indicative of latent infection, but is insufficient as a predictor for the risk of recurrence. As a model for this important medical problem, we compared the risks of murine CMV recurrence from latency established after neonatal primary infection and after infection at adult age. The risk of CMV recurrence was high only after neonatal infection. The copy number of latent viral genome in tissues was identified as the key parameter that determines the overall and organ-specific risks of recurrence. Latent CMV burden and risk of recurrence were related to the extent of virus multiplication during primary infection. The presence of latent CMV in multiple organs provides the molecular basis for stochastic events of recurrence in single organs or in any combination thereof. These findings are discussed as a concept of multifocal CMV latency and recurrence. It provides a rationale for the diversity in the clinical outcome of CMV disease.

Late phase inhibition of murine cytomegalovirus replication by synergistic action of interferon-gamma and tumour necrosis factor

We have shown previously that the antiviral function of CD4+ T lymphocytes against murine cytomegalovirus (MCMV) is associated with the release of interferon-gamma (IFN-gamma). We now demonstrate that IFN-gamma and tumour necrosis factor alpha (TNF-alpha) display synergism in their antiviral activity. As little as 2 ng/ml of IFN-gamma and TNF-alpha reduced the virus yield by about three orders of magnitude. There was no effect on immediate early (IE) and early (E) gene expression as far as the candidate genes IE1, E1 and those encoding the major DNA-binding protein and the DNA polymerase were concerned. Late gene transcription, assayed by the candidate genes encoding glycoprotein B and the MCMV homologue of ICP 18.5, was blocked and MCMV DNA replication was found to be reduced but not halted. The most prominent finding of the cytokine effect, seen by electron microscopy, was an alteration of nucleocapsid formation. Altogether, the synergism is multifaceted and acts at more than one stage during viral morphogenesis. Because the cytokines clearly do not act at an early stage of infection we conclude that the mode of cytokine activity differs between alpha- and betaherpesviruses.

Participation of endogenous tumour necrosis factor alpha in host resistance to cytomegalovirus infection

Interferon gamma (IFN gamma) represents an essential cytokine involved in murine cytomegalovirus (MCMV) clearance from the salivary gland and the control of horizontal transmission. Because IFN gamma cannot be responsible for all cytokine effects during recovery from MCMV infection we have now tested the potential participation of tumour necrosis factor alpha (TNF alpha) in the antiviral defence. Neutralization of endogenous TNF alpha abolished the antiviral activity of CD4 T cells in immunocompetent as well as in CD8 subset-deficient mice. These data suggest that the antiviral effect of the CD4 subset requires the presence of at least two cytokines, namely IFN gamma and TNF alpha. Depletion of endogenous TNF alpha in adoptive cell transfer recipients diminished the antiviral function of CD8 T lymphocytes suggesting that TNF alpha also participates in CD8 T cell effector functions. Furthermore, endogenous cytokines were found to be required for survival after infection with lethal doses of MCMV, whereas immunotherapy with recombinant TNF alpha and IFN gamma could not limit virus replication in vivo. The results suggest that, similar to IFN gamma, TNF alpha is an integral part of the protective mechanisms involved in cytomegalovirus clearance.

Gamma interferon-dependent clearance of cytomegalovirus infection in salivary glands

Cytomegalovirus (CMV), similar to other members of the Herpesviridae family, can establish both persistent and latent infections. Each of the CMVs that are found in many animal species replicates in the salivary gland, and oral secretion represents a source of horizontal transmission. Locally restricted replication characterizes the immunocompetent individual, whereas in the immunocompromised host, protean disease manifestations occur due to virus dissemination. The virus is cleared by immune surveillance, and CD8+ T lymphocytes play a major role. Remarkably, certain cell types of salivary gland tissues are exempt from CD8+ T-lymphocyte control of murine CMV infection and require the activity of CD4+ T lymphocytes. The results presented here suggest that this activity is a function of Th1 cells. Neutralization of endogenous gamma interferon abrogated the antiviral activity of Th1 cells but not that of CD8+ T lymphocytes in other tissues. Neutralization of endogenous gamma interferon did not interfere with the induction of the cellular and humoral immune response but acted during the effector phase. Recombinant gamma interferon could not replace the function of Th1 cells in vivo and had limited direct antiviral activity in vitro. The results therefore suggest that gamma interferon represents one, but not the only, essential factor involved in salivary gland clearance, establishment of CMV latency, and, eventually, the control of horizontal transmission.

Site-restricted persistent cytomegalovirus infection after selective long-term depletion of CD4+ T lymphocytes

We have established a murine model system for exploring the ability of a CD4 subset-deficient host to cope with cytomegalovirus infection, and reported three findings. First, an antiviral response of the CD8 subset of T lymphocytes could be not only initiated but also maintained for a long period of time despite a continued absence of the CD4 subset, whereas the production of antiviral antibody proved strictly dependent upon help provided by the CD4 subset. Second, no function in the defense against infection could be ascribed as yet to CD4-CD8- T lymphocytes, which were seen to accumulate to a new subset as a result of depletion of the CD4 subset. This newly arising subset did not substitute for CD4+ T lymphocytes in providing help to B lymphocytes, and was also not effective in controlling the spread of virus in host tissues. As long as a function of these cells in the generation and maintenance of a CD8 subset-mediated response is not disproved, caution is indicated with concern to an autonomy of the CD8 subset. Third, even though with delay, the CD8+ effector cells raised in the CD4 subset-deficient host were able of clear vital tissues from productive infection and to restrict asymptomatic, persistent infection to acinar glandular epithelial cells in salivary gland tissue.

Molecular basis for cytolytic T-lymphocyte recognition of the murine cytomegalovirus immediate-early protein pp89

The murine cytomegalovirus protein pp89, which is encoded by gene ieI, is a nonstructural regulatory protein expressed in the immediate-early phase of the viral replication cycle and located mainly in the nucleus of infected cells. Protection of BALB/c (H-2d) mice against a lethal murine cytomegalovirus challenge infection is achieved by vaccination with a recombinant vaccinia virus, MCMV-ieI-VAC, expressing pp89 as the only murine cytomegalovirus gene product. The protection is entirely mediated by T lymphocytes of the CD8+ subset. In the present report, we analyzed the molecular basis of the recognition of pp89 by BALB/c CD8+ cytolytic T lymphocytes. A series of internal and terminal deletion mutants of gene ieI was constructed and cloned in vaccinia virus, and the antigenicity and immunogenicity of the fragments of pp89 expressed by the recombinants were studied. A region of only one-sixth of the protein, from amino acids 154 to 249 and encoded by the fourth exon of gene ieI, was sufficient for both the recognition in vitro of the protein by pp89-specific cytotoxic T lymphocytes and the induction in vivo of pp89-specific cytotoxic T lymphocytes. By using synthetic peptides, the sequence between residues 161 and 179, which is located within the defined domain, was identified as an epitope presented to BALB/C cytotoxic T lymphocytes by the class I major histocompatibility antigen Ld.

A nonstructural viral protein expressed by a recombinant vaccinia virus protects against lethal cytomegalovirus infection

The nonstructural immediate-early protein pp89 of murine cytomegalovirus (MCMV) is the first viral protein synthesized after infection and has a regulatory function in viral gene expression. Despite its localization in the nucleus of infected cells, pp89 is also the dominant antigen recognized by MCMV-specific cytolytic T lymphocytes. The recombinant vaccinia virus MCMV-ieI-VAC, which expresses pp89, was used to study the capacity of this protein to induce protective immunity in BALB/c mice. Vaccination with MCMV-ieI-VAC induced a long-lasting immunity that protected mice against challenge with a lethal dose of MCMV but did not prevent infection and morbidity. In vivo depletion of CD8+ T lymphocytes before challenge completely abrogated the protective immunity. CD8+ T lymphocytes derived from MCMV-ieI-VAC-primed donors and adoptively transferred into sublethally irradiated and MCMV-infected recipients were found to limit viral replication in host tissues, whereas CD4+ T lymphocytes and pp89-specific antiserum had no protective effect. The data demonstrate for the first time that a single nonstructural viral protein can confer protection against a lethal cytolytic infection and that this immunity is entirely mediated by the CD8+ subpopulation of T lymphocytes.

Adoptive immunotherapy of murine cytomegalovirus adrenalitis in the immunocompromised host: CD4-helper-independent antiviral function of CD8-positive memory T lymphocytes derived from latently infected donors

The ability of memory T lymphocytes derived from latently infected mice to control murine cytomegalovirus disease in the immunocompromised host was studied by adoptive transfer experiments. At a stage of pathogenesis when virus had already colonized target tissues, a therapeutic antiviral function could be ascribed to the CD8+ subset. This in vivo function was not restricted to sites in which intravenously infused lymphocytes usually are trapped or home in, such as the lungs or the spleen, respectively, but was also evident in the adrenal glands, a site to which antiviral effector cells have to specifically migrate. Specific infiltration of adrenal gland cortical tissue by donor-derived CD8+ memory T lymphocytes was demonstrated. CD4+ memory T lymphocytes had no antiviral effect by themselves and also were not required for the function of the CD8+ effector cells in this short-term immunotherapy model. These findings should help settle the debate about which subset of T lymphocytes comprises the effector cells that can directly control cytomegalovirus infection in the murine model system.

Simultaneous expression of CD4 and CD8 antigens by a substantial proportion of resting porcine T lymphocytes

The existence of four subpopulations in resting porcine T lymphocytes is documented. In addition to the two known subpopulations which are typified by a mutually exclusive expression of either the CD8 or the CD4 differentiation antigen, CD4-CD8+ and CD4+CD8- T lymphocytes, respectively, two unusual subpopulations were prominent not only in peripheral blood, but also in lymphoid tissues: CD4-CD8- T lymphocytes expressing neither of these antigens and CD4+CD8+ T lymphocytes coexpressing both antigens. While CD4+CD8+ lymphoblasts have been found also in other species, resting T lymphocytes with that phenotype are without precedent among all species analyzed to date. This unique composition of the porcine T lymphocyte population has to be taken into consideration when the swine is used as a large animal model in experimental medicine.

Cytolytic T lymphocyte recognition of the murine cytomegalovirus nonstructural immediate-early protein pp89 expressed by recombinant vaccinia virus

The murine immediate-early (IE) protein pp89 is a nonstructural virus-encoded phosphoprotein residing in the nucleus of infected cells, where it acts as transcriptional activator. Frequency analysis has shown that in BALB/c mice the majority of virus-specific CTL recognize IE antigens. The present study was performed to assess whether pp89 causes membrane antigen expression detected by IE-specific CTL. Site-directed mutagenesis has been used to delete the introns from gene ieI, encoding pp89, for subsequent integration of the continuous coding sequence into the vaccinia virus genome. After infection with the vaccinia recombinant, the authentic pp89 was expressed in cells that became susceptible to lysis by an IE-specific CTL clone. Priming of mice with the vaccinia recombinant sensitized polyclonal CTL that recognized MCMV-infected cells and transfected cells expressing pp89. Thus, a herpesviral IE polypeptide with essential function in viral transcriptional regulation can also serve as a dominant antigen for the specific CTL response of the host.

Distribution of T-lymphocyte subsets in porcine lymphoid tissues

The distribution of the functional subsets of porcine T cells, the cytolytic/suppressor (Tc/s) and the helper/inducer (Th/i) cells was studied in cryostat sections of thymus, lymph nodes, tonsils, Peyer's patches, spleen and liver using the indirect immunoperoxidase technique. Three murine monoclonal antibodies (mAb) were used. The mAb 8/1 reacts with an antigen present on all T cells and on cells of the myeloid lineage; the antigen has not yet been characterized biochemically. The mAb 295/33 (anti-T8) binds to the porcine T8 antigen and defines the Tc/s subset, while mAb PT-4 (anti-T4) detects the porcine T4 antigen and defines the Th/i subset. Practically all thymocytes were stained by mAb 8/1. The majority of cortical thymocytes apparently co-expressed T8 and T4, whereas distinct fractions of medullary cells were labelled by either anti-T8 or anti-T4. In peripheral lymphoid organs all three mAb reacted with cells in the thymus-dependent areas and with cells scattered in the lymphoid follicles. In lymph nodes, tonsils and Peyer's patches, anti-T8 and anti-T4 each labelled approximately half of the cells stained by mAb 8/1. In the periarteriolar lymphoid sheath of the spleen, anti-T4 labelled more cells than did anti-T8. The reactivity of mAb 8/1 with the Kupffer cells of the liver demonstrated the expression of the 8/1 antigen on cells of the monocyte lineage. The T8 and T4 antigens could not be detected in acetone-fixed and paraffin-embedded sections, while the antigen recognized by mAb 8/1 remained preserved. Altogether, despite an inverted microanatomical structure of porcine lymph nodes, the frequency and distribution of T8+ and T4+ cells in thymus-dependent areas proved to be similar to those found in other species.

Immune responsiveness in offspring of adoptively immunized mothers

The rate of appearance of cells forming 19S hemolytic antibody (PFC) in the spleens of F1 newborn mice after adoptive anti-paternal immunization of fetuses, newborns and/or their mothers during pregnancy has been studied. An increase in the number of PFC was found at the age of 6 to 11 days in offspring of adoptively immunized mothers. These newborns, even when foster nursed by untreated mothers, still had a significantly higher number of PFC in comparison to the controls. In contrast, adoptive immunization of the newborns themselves resulted in a decrease of PFC during the second week after birth. Several possible explanations for the obtained results are discussed including the putative transplacental passage of immunocompetent cells.

Monoclonal antibodies reactive with swine lymphocytes. I. Antibodies to membrane structures that define the cytolytic T lymphocyte subset in the swine

A panel of monoclonal antibodies (mAb) with specificity for swine leukocytes was prepared by somatic cell hybridization with the use of spleen cells from mice immunized with swine thymocytes. The reactivity of two mAb (295/33 and 122/28), which both immunoprecipitated from the surface of swine leukocytes an antigen termed S-L2 with an apparent m.w. of 33 to 35 kilodaltons under reducing and 65 to 70 kilodaltons under nonreducing conditions, was investigated in detail. These mAb were reactive in indirect immunofluorescence with 50 to 60% of thymocytes, 35% of peripheral blood lymphocytes, and 55% of E rosette-positive cells; they were nonreactive with bone marrow cells, Ig+ B cells, nonrosetting lymphocytes, granulocytes, and monocytes. In functional studies, the elimination of S-L2+ cells partially reduced the proliferative response to concanavalin A and pokeweed mitogen but not to Staphylococcus aureus and lipopolysaccharide. The S-L2- subset proliferated well to alloantigens. Both cytolytic T effector cells and precursor cells carried the antigen S-L2 and could be depleted from heterogeneous cell populations by both antibodies in the presence of complement. These data suggest that the mAb 295/33 and 122/28 recognize a specific polypeptide present on the surface of swine cytolytic T cells. These antibodies will be useful in studies on the swine immune system.

Monoclonal antibodies reactive with swine lymphocytes. I. Antibodies to membrane structures that define the cytolytic T lymphocyte subset in the swine

A panel of monoclonal antibodies (mAb) with specificity for swine leukocytes was prepared by somatic cell hybridization with the use of spleen cells from mice immunized with swine thymocytes. The reactivity of two mAb (295/33 and 122/28), which both immunoprecipitated from the surface of swine leukocytes an antigen termed S-L2 with an apparent m.w. of 33 to 35 kilodaltons under reducing and 65 to 70 kilodaltons under nonreducing conditions, was investigated in detail. These mAb were reactive in indirect immunofluorescence with 50 to 60% of thymocytes, 35% of peripheral blood lymphocytes, and 55% of E rosette-positive cells; they were nonreactive with bone marrow cells, Ig+ B cells, nonrosetting lymphocytes, granulocytes, and monocytes. In functional studies, the elimination of S-L2+ cells partially reduced the proliferative response to concanavalin A and pokeweed mitogen but not to Staphylococcus aureus and lipopolysaccharide. The S-L2- subset proliferated well to alloantigens. Both cytolytic T effector cells and precursor cells carried the antigen S-L2 and could be depleted from heterogeneous cell populations by both antibodies in the presence of complement. These data suggest that the mAb 295/33 and 122/28 recognize a specific polypeptide present on the surface of swine cytolytic T cells. These antibodies will be useful in studies on the swine immune system.