Cell-mediated immunity to moloney sarcoma virus in mice. I. Analysis of antigens responsible for lymphocyte stimulation in regressor mice

Purified viruses, viral antigens, and cell extracts have been tested for their ability to stimulate protein synthesis by MSV-M-1 regressor spleen cells from BALB/c mice. Immune, but not normal cells, responded to MSV-M, but not to MSV-G virus-1, and to the type-specific viral envelope glycoprotein from MSV-M virus. Extracts of mouse embryo fibroblasts transformed by either MSV-M or MSV-G, however, specifically stimulated MSV-M regressor spleen cells. Cells stimulated by different antigens, and by phytohaemagglutinin, had the same sedimentation profile and were identified as T-lymphocytes.

Immunity to murine sarcoma virus induced tumours. 3. Analysis of the cell populations involved in protection from lethal tumour progression of sublethally irradiated, MSV inoculated, mice

A comparison was made between the cells responsible for demonstrable activity against MSV antigens, using both in vivo and in vitro assays. Similar cells (in terms of size and sensitivity to anti-theta serum) were detected in both assays. However, while lymphoid cells from animals at all stages post-MSV infection were active in protecting irradiated mice from the lethal effect of induction of MSV sarcomata, cells from animals at early stages post-MSV infection (when the tumour was in a progressive phase of growth) were not active in the in vitro assay. By manipulation of the in vivo assay conditions a situation was observed in which cells from "progressor animals" were able to suppress both the in vitro and in vivo activity of regressor lymphoid cells. The potential physiological role of this cell type is disussed.

Separable populations of activated thymus-derived lymphocytes identified in two assays for cell-mediated immunity to murine tumor allografts

The immune response of C57BL mice to a DBA/2 tumor allograft has been assessed in two assays of cell-mediated immunity, the in vitro lysis of (51)Cr-labeled target cells and the antigen-mediated inhibition of macrophage migration. Both assays were shown to be measuring a T-cell-mediated reaction. Three types of experiments suggested that distinct subpopulations of T cells mediate these reactions. The tissue distributions of these activities was distinctive; both activities were present in spleens from i.p. immunized mice, but only macrophage migration inhibition activity was found in the peripheral lymph nodes (PLN) of such mice. Adoptive transfer of immune spleen cells into irradiated syngeneic recipients revealed that while a substantial amount of migration inhibition activity could subsequently be found in PLN, cytotoxic activity was found predominantly in the spleens of these adoptive hosts. Velocity sedimentation analysis of immune cells 14 days after i.p. immunization indicated that while the majority of cytotoxic activity was associated with small and medium lymphocytes, the majority of migration inhibition activity was associated with medium and large lymphocytes. In addition, normal spleen cells were fractionated by velocity sedimentation immediately before allosensitization in vitro. Subsequent analysis of the sensitized fractions revealed that the activity profiles for cytotoxicity and macrophage migration inhibition were not coincident. The implications of these observations are discussed.

Immunity to Murine Sarcoma Virus-Induced Tumors

Experiments have been performed to examine the immune status in animals carrying tumors which are in their progressive phase of growth. Unfractionated spleen or lymph node cells of BALB/c mice, inoculated 10 to 14 days previously with murine sarcoma virus (MSV), have no demonstrable immunity in vitro to MSV-viral antigens. On fractionation of the spleen cells from such animals, however, large and small cells can be detected which react in well defined assays for immunity in this system. A cell population of intermediate size is capable of suppressing this activity.

Treatment of the suppressing cell population with anti-θ or anti-immunoglobulin antiserum has provided data to suggest that the suppression is caused by a cell with immunoglobulin moieties on its surface. Moreover, evidence is presented to show that this suppression is nonspecific, in that the induction of protein synthesis in thymus-associated (T) cells in response to the mitogen phytohemagglutinin (PHA) is also suppressed by a cell with the above characteristics. The significance of these findings in relation to the status of growth of the tumor in the host is discussed.

Immunity to Murine Sarcoma Virus-Induced Tumors

BALB/c mice immunized with murine sarcoma virus (MSV) have been studied at various times after injection for their immunity to either extracts of MSV-induced sarcomas or preparations of disrupted whole MSV virus. The assays used were the inhibition of normal macrophage migration, in the presence of immunizing antigen and sensitized cells, or the stimulation of protein or DNA synthesis in immune cells themselves, caused by the addition of the immunizing antigen.

Each assay system is shown to detect specific anti-MSV-induced immunity in both the spleen and lymph node of infected mice. The immune cells causing the effects observed in the systems tested seem to be T lymphocytes, as judged by the effect of anti-θ antiserum. Evidence is presented that two physically distinct populations of cells exist which can respond to MSV and show a net increase in total DNA or protein synthesis. However, only one of these cells seems able to release factors able to cause macrophage migration inhibition.

Evidence for in Vivo Protection against Murine-Sarcoma Virus-Induced Tumors by T Lymphocytes from Immune Animals

Normal 3- to 4-week old BALB/c mice inoculated with murine sarcoma virus (MSV) develop sarcomas within 7 to 10 days, which generally regress by day 15 to 20. Sublethal irradiation of the recipients, prior to inoculation of MSV, leads to the development of progressively growing tumors and ultimate death of the host within 30 days. Small numbers (1 × 107) of spleen cells from mice whose tumor has regressed, but not similar numbers of cells from age-matched control mice, protect these irradiated mice from developing progressively growing tumors. Pretreatment of the protecting cells with anti-θ antibody and complement, but not with anti-immunoglobulin antibody and complement, abolishes the protective effect.

Specific modulation of antibody production in vitro by soluble mediators

The in vitro 19S-PFC response to SRBC reaches a peak at 4 days. After this time, the number of 19S-PFC gradually decreases with a half-life in vitro of approximately 20 hours. The factors which influence this rate of decline of 19S-PFC after 4 days in vitro have been investigated. The decline is not the result of non-specific inhibitors in the tissue medium: medium harvested from cultures at the time of the peak PFC response can support another PFC response by fresh spleen cells. The survival of 19S-PFC in culture is a sensitive function of the number of specific T cells in the culture, since removal of T cells enhances the decline of PFC. Evidence is presented that a θ-negative cell with surface receptors for SRBC can specifically enhance the rate of decay of SRBC-specific 19S-PFC in vitro. This decay is mediated by an antigen-specific soluble mediator which cannot be absorbed by specific antigens. This factor is unlikely to be specific hyperimmune mouse IgG since addition of anti-SRBC IgG at 4 days has no affect on the rate of decay of PFC. Although the properties of the supernatant activity have not been characterized, it might be either an anti-receptor antibody or an antigen—antibody complex.

Stimulation of Early Protein Synthesis as an Assay of Immune Reactivity: Analysis of the Cells Responding to Mitogens and Alloantigens

Spleen cells from nonimmune mice were cultured for 24 hr in the presence of phytohemagglutinin (PHA), lipopolysaccharide (LPS), or alloantigens. Data are presented to show that within this time period all three stimuli caused an increase in protein synthesis. The response to PHA and alloantigens was defined as a T cell response by virtue of the effects of anti-θ antibody, adult thymectomy, and velocity cell sedimentation. Similarly, the response to LPS was determined to be a B cell response since it was unaffected by anti-θ antibody or adult thymectomy but was abolished by treatment of responder cells with anti-Ig antiserum and complement. Furthermore, the cells responding to LPS were physically separable from those responding to PHA and alloantigens and sedimented with a velocity characteristic of B lymphocytes.

In vivo requirement for a radiation-resistant cells in the immune response to sheep erythrocytes

Experiments have been done to establish whether the radiation-resistant or A cell has a specific function in the initiation of an immune response in mice to sheep erythrocytes (SRBC). All previous demonstrations using accessory (A) cells have involved in vitro assays and are possibly explainable as tissue culture artifacts. If A cells are essential, it should be possible to demonstrate their requirement in vivo. Therefore we first established such conditions. Two methods were found for creating an A-cell deficiency in vivo: (a) A cells disappear gradually from the spleens of irradiated mice, presumably by migration since A-cell function was shown not to be decreased by irradiation. If 3 days elapse between irradiation and transplantation of mixtures of bone marrow and thymus cells (which provide B and T but few A cells), the usual synergistic response does not occur. Addition of large numbers of freshly irradiated spleen cells to the mixture of bone marrow and thymus completely restores the immune response. (b) Injection of 10(10) horse erythrocytes into mice suppresses A-cell activity in these mice 24 hr later; a much reduced response to SRBC is obtained when they are given at this time. The response can be partially restored if irradiated spleen cells are given with the SRBC. This observation formed the basis for a quantitative in vivo assay for A cells in which the magnitude of restoration by various suspensions of irradiated cells was used to estimate the A-cell activity of that suspension. A quantitative in vitro assay for A cells was also developed. It was essential for this assay that the total cell number, B-cell number, and T-cell number be kept constant and that only the number of A cells be allowed to vary. Only under these conditions was the response a linear function of the number of A cells added. If the in vivo and in vitro assays are detecting the same class of radiation-resistant cells, the physical properties of the cells active in each assay should be identical. Spleen cells were separated on the basis of both density and sedimentation velocity. Fractions from both separation methods were tested for their content of A cells using both the in vivo and in vitro assays. The density and sedimentation profiles of A cells were similar in both assays. The demonstration that a radiation-resistant cell is required in vivo and that this cell has properties identical to the radiation-resistant cell required in vitro indicates that this cell (the A cell) is directly involved in the initiation of an immune response to erythrocyte antigens.

Identification by density separation of antigen-specific surface receptors on the progenitors of antibody-producing cells

Initiation of an immune response to sheep erythrocytes by the transplantation of spleen cells into irradiated recipients is thought to involve an interaction between two functionally different lymphoid cells. If such populations actually exist, it should be possible to separate them from each other and to study the properties of each type of cell. To this end, mouse spleen cells have been fractionated using equilibrium centrifugation in density gradients of Ficoll. Two functionally different populations of spleen cells were obtained. Neither population was active in initiating an immune response by itself on transplantation into irradiated mice, but mixtures of the two populations were. One population of spleen cells was also active when transplanted together with normal mouse bone marrow and the other population was active when mixed with thymus cells from normal mice. These results indicate that two different spleen cells must interact to initiate an immune response in vivo.

The specificity of the cell that synergizes with thymus cells was investigated further and shown to have antigen-specific receptors on its surface. These cells can be recognized by their ability to make rosettes with erythrocyte antigens. Thus, the `background' rosette-forming cells found in unimmunized mice are actually progenitors of antibody-producing cells.

Homogeneity of antibody-producing cells as analysed by their buoyant density in gradients of Ficoll

Ficoll, a polymer of sucrose, has several advantages over proteins for preparation of density gradients used to study mammalian cells. In addition to its ease of preparation, Ficoll as a result of being an uncharged molecule, does not bind ions from solution and, therefore, does not affect the osmolality of the medium in which it is dissolved.

In studies on antibody-producing cells we found that the measurement of buoyant density in Ficoll at pH 7.2 gives an accurate measurement of cell density. In such gradients antibody-producing cells band in a single peak at a density of 1.070 g/cm³. Two factors have a profound influence on the density profile of antibody-producing cells. Lowering the pH to 5.5 lowers their buoyant density to 1.062 g/cm³. Creation of an inverse osmotic gradient varying from 294 m-osmolal at the top of the gradient to 286 m-osmolal at the bottom splits the single peak of antibody-producing cells into three distinct peaks. This effect of osmotic gradients could account for the multiple peaks observed when cells are separated on protein gradients.