Naturally occurring spleen-associated suppressor activity of the newborn mouse. Requirement for two genetic restrictions in suppression of lethal graft-versus-host disease by newborn spleen cells

The identification of the neonatal NBXFO hybridoma cell and its mediator

The neonatal spleen:myeloma hybridoma cell, NBXFO, with immunosuppressive properties supported rodent hematopoietic colony formation. We identified this hybridoma to be an undifferentiated fibroblast that produced macrophage colony-stimulating factor (M-CSF). The bone marrow cells that grew in the presence of the NBXFO supernate were macrophages and were immunosuppressive towards lymphocytes. Neutralizing anti-M-CSF antibody partially inhibited the actions of the neonatal splenic suppressor cells. Neonatal splenocytes, but not the other parental cell line, FO, induced macrophage colony formation, possessed surface-associated M-CSF, and possessed M-CSF-specific transcripts. Therefore, we believe that the M-CSF-producing phenotype was contributed by a fibroblastic stromal cell and that these stromal cells could be responsible for the in situ generation of neonatal splenic suppressor cells.

Neonatal thymic contrasuppressor activity on graft versus host reactions towards self histocompatibility antigens. Parental effects

The ability of fetal and neonatal F1 thymocytes to regulate parental graft versus host (GvH) reactions against self histocompatibility antigens was investigated. The results obtained showed that: (1) fetal F1 thymocytes were able to suppress both maternal and paternal GvH reactivity; (2) at birth, thymocytes were still able to suppress maternal GvH reactivity while no suppression of paternal reactions was detected; the ability to suppress maternal GvH reactions could be detected until day 3; (3) the loss of suppressor activity correlated with the ability of thymocytes to contrasuppress parental GvH reactions. Thus, 24-h F1 thymocytes showed contrasuppressor activity on paternal GvH reactivity and 4-day thymocytes on maternal reactivity. Thymic cells with contrasuppressor activity were shown to be Lyt-1+, CD4+, CD8- and adherent to Vicia villosa. These results suggest the existence of parental effects influencing the duration of thymic suppression and the subsequent appearance of contrasuppressor activity on GvH reactions against self histocompatibility antigens, according to the maternal or paternal origin of self antigens towards which the reaction is directed.

Natural suppressor cells in spleens of irradiated, bone marrow-reconstituted mice and normal bone marrow: lack of Sca-1 expression and enrichment by depletion of Mac1-positive cells

We have recently reported the development of natural suppressor (NS) cells in lethally irradiated, bone marrow-reconstituted mice during the early weeks after bone marrow transplantation (BMT). These cells were shown to be derived primarily from the syngeneic marrow component in recipients of mixed allogeneic plus syngeneic (host type) marrow, and it was speculated that they might be responsible for the anti-GVHD effect previously described for T-cell-depleted syngeneic marrow. It was therefore of interest to look for such suppressive activity in normal adult bone marrow, which might serve as an obtainable source of such cells if they were to be isolated and used clinically. Such activity has indeed been found in normal adult bone marrow and its characteristics compared to that in spleens of early BMT recipients. Suppressive cells from both sources were similar in their specificity patterns and radiosensitivity, and were of the null (i.e., non-T, non-B, nonmacrophage) cell phenotype. Suppression from either source can be enriched by removal of Mac1-positive cells, providing a possible approach to obtaining NS-enriched populations for in vitro expansion and adoptive transfer studies. Such depletion of Mac1-positive cells was associated with a threefold enrichment of Thy1-positive cells, of which one half were CD4- and CD8-negative, similar to the reported phenotype of cultured NS cell lines. Even when enriched in this manner, the contribution of Thy1-positive cell populations did not reach statistical significance. A recent report has suggested that NS cells might actually be pluripotent hematopoietic stem cells. In contrast, we report here that depletion of Sca1-positive pluripotent hematopoietic stem cells with monoclonal antibody plus immunomagnetic beads does not remove NS activity.

A comment on H-2-restricted T cell recognition of Mls determinants: a question of perspective

Data presented here addressed the question of whether T lymphocyte recognition of Mls determinants is MHC restricted. Using isolated T cell blasts from primary MLCs involving H-2-identical, Mls-disparate strain combinations as responding cell populations in secondary MLCs, and measuring their responses against fresh stimulating cells from different strains, has permitted the study of reactions towards Mls determinants in the presence of MHC antigenic differences. Results strongly suggest that recognition of Mlsc (Mls-2 locus) determinants is H-2 restricted. In contrast, there is still little evidence that the recognition of Mlsa (Mls-1 locus) determinants is H-2 restricted. Furthermore, data are also presented which refute the notion that different H-2 haplotypes possess different abilities to present Mlsa determinants to responding T cells.

Soluble factors secreted by naturally occurring suppressor cells that interfere with in vivo graft-vs.-host disease and with T cell responsiveness in vitro

A potent immunosuppressive factor (SUF) is found in the supernatant of short-term cultures of unstimulated thymocytes or spleen cells of neonatal mice and rats and in culture medium of hybridoma cell lines established by fusing neonatal mouse spleen cells with T lymphoma cells (the BW 5147 line). In vitro incubation of spleen cells with SUF suppresses the acute in vivo graft-vs.-host disease, normally induced by allogeneic spleen cells in lethally irradiated mice. Incubation of bone marrow cells with SUF does not affect the hemopoietic stem cells. The addition of SUF to mixed lymphocyte reaction cultures strongly suppresses lymphocyte proliferation. The non-species-restricted inhibition of cell proliferation induced by SUF is shown not to be due to toxicity or nonspecific interference with DNA synthesis. Molecular size fractionation of crude SUF revealed two active moieties: a large moiety of molecular mass greater than 100 kDa and a small moiety of less than 3 kDa. The high kDa moiety mediates T cell unresponsiveness both in vivo and in vitro. In vitro studies revealed that this moiety primarily affects an early event in the proliferative response to alloantigen and mitogen, that prevents interleukin 2 (IL 2) receptor expression and, consequently, blastogenesis and DNA duplication. It does not affect, however, the synthesis of IL 2. The suppressive activity of the low kDa moiety can be demonstrated only in in vitro systems. Pre-treatment of donor lymphocytes with this fraction cannot prevent graft-vs.-host disease mortality. The inhibition of cell proliferation induced by this fraction in vitro is most likely due to interference with the utilization of IL 2, as suggested by its suppressive effect on the proliferation of CTLL-2 cells (an IL 2-dependent cell line).

Neonatal cells in the immunoregulation of parental alloreactivity

The ability of neonatal murine F1 cells to regulate parental graft vs. host (GvH) reactions was investigated. Neonatal F1 splenocytes were able to decrease significantly the deleterious effects of systemic GvH reactions induced either with maternal or paternal splenocytes in a third party strain. Both neonatal F1 splenocytes or thymocytes were able to decrease local GvH reactions induced with maternal splenocytes towards paternal histocompatibility antigens. In the same experimental conditions, however, neonatal F1 cells were unable to decrease local GvH reactions induced with paternal splenocytes towards maternal histocompatibility antigens; using different numbers of neonatal F1 cells not only was no suppressive effect detected but even, a significant increase in GvH was registered. Similar results were obtained when mortality assays were carried out. It can be concluded that neonatal F1 mice differ in their capacity for regulating parental alloreactive T reactions against self histocompatibility antigens either of maternal or paternal origin.

Naturally occurring, spleen-associated suppressor activity of the newborn mouse. Biochemical and functional identification of three monokines secreted by newborn suppressor-inducer monocytes

Immature monocytes residing in the spleens of newborn mice less than 6-7 days of age are known to suppress T-dependent and T-independent immune responses in vitro and in vivo. Suppression is mediated in part through products secreted by the newborn monocytes. In the present study, we have identified 3 monokines secreted by newborn, but not adult, monocytes correlating with materials possessing significant suppressor activity in vitro. These monokines have molecular weights of 58, 10.8, and 10 kilodaltons, appear to be proteins, and are insensitive to heat inactivation. The 58 kd material is antigenically distinct from alpha-fetoprotein, while the lower molecular weight materials are too large to be prostaglandins. Although these monokines can be shown to have activity in vitro, they are not as efficient as newborn monocytes in preventing alloreactivity in vivo.

Consequences of cyclophosphamide treatment in murine lymphocytic choriomeningitis: evidence for cytotoxic T cell replication in vivo

Administration of a large dose (400 mg/kg) of cyclophosphamide (Cy) prior to infection with lymphocytic choriomeningitis virus (LCMV) suppresses the virus-immune cytotoxic T lymphocyte (CTL) response. Treatment with 150 mg/kg of Cy before, at the time of, or on the day after LCMV infection has little effect on CTL function. In contrast, when given on 2-7 days after LCMV, this dose profoundly depresses the day-8 CTL response and delays the onset of neurological disease. Administration of as little as 50 mg/kg of Cy has a marked effect on CTL activity when given 5 days after virus. Therefore it seems likely that CTL are replicating for 5 or 6 days during this primary response. Multiplication of these T cells is apparently completed by day 8, since inoculation of 150 mg/kg of Cy at this time has little effect on the level of CTL activity measured on day 9. The CTL activity is not reconstituted by in vitro culture with added helper factors. The interpretation that CTL are replicating in vivo following inoculation of LCMV is in accord with other analyses of virus-specific precursor frequency in primed and naïve mice.

Cloned natural suppressor cell lines derived from the spleens of neonatal mice

The establishment and characterization of cloned natural suppressor (NS) cell lines derived from the spleen of neonatal BALB/c mice are described. Cloned NS cells suppress the mixed leukocyte reaction (MLR) between normal adult responder and stimulator spleen cells with a 50-fold greater efficiency than fresh neonatal cells. Suppressive activity of both cells did not depend on the haplotype of the responder or stimulator cells, and was radioresistant. Cloned NS cells did not inhibit the uptake of [3H]thymidine by HT-2 cells proliferating in response to interleukin 2 (IL-2), nor the in vitro secretion of IL-1 by macrophages in response to lipopolysaccharide. Several experiments indicated that absorption of IL-2 could not explain the suppression of the MLR by the NS cells in the range of cell numbers tested. The results suggest that NS cells may suppress the MLR by interfering with early stages of T cell activation. The cell surface of a cloned NS cell line was examined using immunofluorescence staining, and was strongly positive for the Thy-1.2, Ly-5, and asialo-GM1 antigens. However, Lyt-1, Lyt-2, surface Ig, IE, MAC-1, and Fc and C3 receptor markers were not detected. In addition, NS cells showed no cytolytic activity against the YAC-1 target cell line. On the basis of these findings, cloned NS cells do not appear to be mature T cells, B cells, macrophages, or NK cells. The development of cloned NS cells may be useful in determining the identity and mechanism of action of nonspecific suppressor cells in the neonatal spleen, and their role in neonatal tolerance and maternal-fetal relationships.