Suppressive effect of leukemia virus-infected lymphoid cells on in vitro immunization of normal splenocytes

Non-toxic endotoxin polysaccharide induces soluble mediators which potentiate antibody production by murine retrovirus-suppressed splenocytes

Mice infected with Friend leukemia virus show marked acquired immunodeficiency characterized by the impairment of immune function of spleen cells to various antigens, both in vivo and in vitro. The large mol. wt. endotoxin derived from Serratia marcescens, as well as a smaller non-toxic polysaccharide derivative, were found to augment the antibody responsiveness of spleen cells from normal as well as FLV-infected mice. In addition, serum from normal donor mice pretreated with BCG and injected either with endotoxin or the polysaccharide derivative potentiated the antibody response of spleen cells from both normal and FLV-infected mice. Similar enhancement was induced by "antibody response helper factor(s)" present in 3-5 day spleen culture supernatants from endotoxin or polysaccharide-treated spleen cells from normal mice. Enhancement of the antibody response of spleen cells from FLV-infected mice by the antibody helper activity was due to stimulation of B-lymphocytes and reversal of a defect in antibody helper factor(s) formation by macrophages. Similar antibody response enhancing activity was induced by both endotoxin and the non-toxic polysaccharide derivative in cultures of normal spleen cells, adherent spleen cell populations, peritoneal cells and the P388D1 macrophage cell line.

Immunotherapy of murine leukemia. XI. Differential susceptibility of spleen cells from serum-protected mice to thein vitro immunosuppressive effects of friend leukemia virus-infected splenocytes

It has previously been shown that DBA/2 mice protected against the development of Friend leukemia virus (FLV)4-induced disease by the passive administration of heterologous antisera directed against disrupted virions or the major viral envelope glycoprotein (gp71) fail to undergo the generalized immunosuppression which characterizes FLV leukemogenesis. In the present studies, the susceptibility of spleen cells from serum-protected mice to the immunosuppressive effects of FLV-infected spleen cells has been examined by means of in vitro assays of antibody production and of natural killing (NK). In contrast to the parallel suppression of both functions in FLV-infected mice and their lack of suppression in serum-protected animals, a dichotomy was observed in the in vitro susceptibility of these activities of spleen cells from serum-protected mice to the suppressive effects of virus-infected splenocytes, implying that more than one mechanism of suppression is operative. Thus, the antibody-producing capability of serum-protected splenocytes was not suppressed by FLV-infected spleen cells, this suppression reflecting the activity of infectious virus, while the NK effector function of serum-protected spleen cells was as susceptible as that of normal splenocytes to suppression by virus-infected spleen cells. This suppression of NK activity is mediated by virus-induced suppressor cells and does not involve free infectious virus, in contrast to suppression of antibody production. These results thus indicate that, while serum therapy inhibits the development of the virus-induced NK suppressor cells found in the spleens of progressively infected mice, splenic NK effectors, which are present, demonstrate full susceptibility to the suppressive effects of such independently generated virus-induced suppressor cells. Preliminary attempts at characterizing these NK suppressors in the spleens of FLV-infected mice indicate that they are not eliminated by treatment with antisera directed against T cells, B cells, macrophages or FLV gp71. However, Percoll fractionation of spleen cells from infected mice has revealed the presence of an expanded (as compared to normal spleen cells) population of cells of density 1.077 which demonstrates high in vitro NK suppressor activity, presumably representing the virus-induced suppressor cells measured in this assay. At the present time, the mechanism of the resistance of serum-protected spleen cells' antibody production to suppression by FLV-infected splenocytes (i.e., by FLV itself) remains undefined.

Role of antibody response helper factors in immunosuppressive effects of friend leukemia virus

The primary antibody response of BALB/c splenocytes to sheep erythrocytes in vitro was suppressed by infection with Friend leukemia virus (FLV), with the response capacity decreasing with increasing duration of infection. The acquisition of normal antibody responses was amplified by macrophage-produced antibody response helper factor(s). FLV-infected mice were treated with bacterial lipopolysaccharide to induce the release of these helper factors into the serum. Similar to the loss of antibody response capacity by their splenocytes, the FLV-infected mice progressively lost the ability to produce helper factors in response to lipopolysaccharide. In vitro cultures of FLV-infected cells also showed a depressed ability to produce helper factor activity both spontaneously and in response to lipopolysaccharide stimulation. The reconstitution of normal levels of exogenous helper factors to FLV-infected splenocytes restored the antibody response to normal or even elevated levels. These studies indicate that the mechanism for suppression of antibody responses by FLV involves the depression of antibody response helper factor production.

Influence of viruses on cells of the immune response system

A wide variety of immune parameters may be influenced in vivo and in vitro during virus infections. Some viruses appear to specifically alter macrophage activity, while other viruses preferentially affect activity of lymphocytes, both B and T cells. There are many mechanisms involved in immune derangement during virus infection, including the possibility that viruses preferentially interact with selected classes of immunocompetent cells. Alternately, some of the events associated with viral derangement of the immune response may be associated with the release or formation of factors by cells infected with a virus which affects other cells of the immune system. Similarly, the development of immune complexes resulting from the interaction of viruses or their components with antibody directed to the virus may influence the immune response, as well as induce other immunopathologic manifestations. Many model systems have been studied, especially those with viruses important to human infection. It is noteworthy that leukemia viruses and, in some cases, other tumor associated viruses may directly alter the immune response, usually before other manifestations of the tumorigenic process is evident. Tumor virus-induced immunomodulation is associated with direct effects of a virus on immunocompetent cells per se as well as the formation of soluble factors induced by the virus infection. These and other considerations indicate that interactions of viruses with the host immune defense system are an important consideration in understanding how virus infection is initiated and progresses, and how such infection affects the host's physiologic activity, including immunocompetence, per se.

Factors from lymphoid cell tumor affecting immune responses

Friend leukemia virus induces erythroblastic leukemia in genetically susceptible BALB/c mice. FLV-containing leukemic cells markedly depressed the humoral immune response to SRBC in the appropriate mouse strain. Both immunosuppression and leukemogenesis were readily transmitted by cell-free virus-containing homogenates of the FLV leukemic splenocytes into normal BALB/c mice. In the present study it was found that both Friend leukemic splenocytes as well as virus containing extracts from the leukemic cells were neutralized by heating and by specific antisera. Suppressive activity passed through a 0.45 mu filter but not a 300,000 MW filter and could be pelleted at 100,000 x g. They were also highly resistant to inactivation by irradiation. Mice given leukemic splenocytes after irradiation with up to 32.000 rads still developed leukemia. Addition of either normal or irradiated FLV-leukemic cells to normal spleen cell cultures in vitro markedly suppressed antibody formation. At least 32,000 rads were required to significantly impair the immuno-suppressive activity of the FLV-leukemic cells. Thus, virus per se appears to be directly responsible for suppression of antibody formation to FLV.

Stimulation by a hydroxythiazolobenzimidazole of enhanced formation of antibodies to sheep erythrocytes in vitro and in vivo

A hydroxythiazolobenzimidazole, a low molecular weight compound, was found to have an immunoenhancing effect on both the in vivo and in vitro antibody response of mouse spleen cells to an optimum immunizing dose of sheep red blood cells. At noncytotoxic concentrations the optimum range of 25 to 50 micrograms per 5 x 19(6) spleen cells was most effective in vitro. Concentrations greater than 100 micrograms per culture were toxic in vitro and reduced cell viability as well as antibody responsiveness. The compound enhanced to an even greater degree the antibody response of spleen cell cultures immunized with suboptimum doses of SRBC. The background PFC response, in the absence of SRBC, also was stimulated by the benzimidazole. These immunoenhancing responses were not related to mitogenic effects, since increased thymidine uptake did not occur when normal mouse spleen cells were incubated with graded doses of the compound. Therefore, the immunostimulatory properties of the compound, both in vivo and in vitro, were not due to mitogenic stimulation of lymphoid cells.

Scanning electron microscopy of lymphoid cells from leukemia virus-infected mice

Spleen, lymph node, bone marrow, and thymus cells from Friend leukemia virus (FLV)-infected mice were examined by scanning electron microscopy. Whereas splenocytes from normal, noninfected animals showed the expected morphological classes of lymphocytes, including cells with numerous villous projections and smoother cell types, spleen cells from mice infected with FLV showed a rapid alteration of surface morphology. Shortly after infection, a decrease in the number and percentage of villous cells occurred, with a concomitant increase in the number of cells that were larger and smoother. Within 10 to 20 days after infection, the majority of splenocytes were smooth, large cells showing many distinct morphological charges, including surface "holes" and a "spongy" appearance. By days 25 to 35 after infection, most splenocytes were abnormal in appearance. Similar changes occurred in the lymph nodes after FLV infection, but the rate of change was much lower. Abnormal and larger smooth-surfaced cells did not become prominent until after week 2 or 3 infection. Thymus and bone marrow cells showed little if any change in surface morphology until late in the infectious process. However, even at that time only a few of the cells were abnormal in appearance. The changes in cell population in the spleen but not the lymph nodes paralleled the rapid decrease in the percentage of cells which stained positive for surface immunoglobulin and theta antigen. Futhermore, FLV antigen rapidly appeared on spleen cells after infection; fewer lymph node cells were positive, and only low numbers of marrow and thymus cells stained positive for FLV antigen. The marked immunosuppression induced by FLV infection paralleled and in some instances preceded the marked morphological changes.

Murine lymphoma-induced immunosuppression: requirement for direct tumour cell contact

The FBL-3 lymphoma cell line caused impaired antibody formation in vivo when injected into mice intraperitoneally, and in vitro when added to normal syngeneic spleen cells immunized in vitro with sheep erythrocytes. Immunosuppression occurred only when intact viable tumor cells were cocultivated with the normal spleen cells. As few as 10(5) FBL-3 cells, when added to 5 X 10(6) normal cells, impaired antibody formation. However, cell-free extracts of filtrates from even much larger numbers of tumor cells did not affect antibody formation, either in vitro or in vivo. Heating the tumor cells at 56 degrees C or irradiation with as little as 1000 rads completely abolished immunosuppressive activity, both in vitro and in vivo. Separation of viable tumor cells from target antibody-forming cells by cell-impermeable membranes prevented immunosuppression, showing that direct cell-to-cell contact is required for immunosuppression.

Mechanisms of genetic resistance to Friend virus leukemia. III. Susceptibility of mitogen-responsive lymphocytes mediated by T cells

Friend leukemia virus (FV) suppressed the proliferative responses of spleen, lymph node, marrow, and thymus cell populations to various T- and B-cell mitogens. Cells taken from mice, e.g. BALB/c genetically susceptible to leukemogenesis in vivo were much more susceptible to suppression of mitogenesis in vitro than similar cells from genetically resistant mice, e.g., C57BL/6. Nylon wool-purified splenic T cells from BALB/c and C3H mice lost susceptibility to FV-induced suppression of mitogenesis but became suppressible by addition of 10% unfiltered spleen cell. Thus, FV mediates in vitro suppression of lymphocyte proliferation indirectly by "activating" a suppressor cell. The suppressor cell adhered to nylon wool but not to glass wool or rayon wool columns. Pretreatment of spleen cells with carbonyl iron and a magnet did not abrogate the suppressor cell function. Suppressor cells were not eliminated by treatment with rabbit antimouse immunoglobulin (7S) and complement (C). However, high concentrations of anti-Thy-1 plus C destroyed suppressor cells of the spleen; thymic suppressor cells were much more susceptible to anti-Thy-1 serum. Nude athymic mice were devoid of suppressor cells and their B-cell proliferation was relatively resistant to FV-induced suppression in vitro. The suppressor cells in the thymus (but not in the spleen) were eliminated by treatment of mice with cortisol. Thus, FV appears to mediate its suppressive effect on mitogen-responsive lymphocytes by affecting "T-suppressor cells." Spleen cells from C57BL/6 mice treated with 89Sr to destroy marrow-dependent (M) cells were much more suppressible by FV in virto than normal C57BL/6 spleen cells. However, nylon-filtered spleen cells of 89Sr-treated C57BL/6 mice were resistant to FV-induced suppression in vitro, indicating that the susceptibility of spleen cells from 89Sr-treated B6 mice is also mediated by suppressor cells. Normal B6 splenic T cells were rendered susceptible to FV-induced suppression of mitogenesis by addition of 10% spleen cells from 89Sr-treated B6 mice. Thus, M cells appear to regulate the numbers and/or functions of T-suppressor cells which in turn mediate the immunosuppressive effects of FV in vitro. Neither mitogen-responsive lymphocytes nor T-suppressor cells are genetically resistant or susceptible to FV. The genetic resistance to FV is apparently a function of M cells, both in vitro as well as in vivo.

Cellular events involved in the true primary immune response of splenocytes in vitro

An antibody response showing characteristics of a "true" primary response was readily induced in vitro with splenocytes from normal non-immunized animals cultured with strongly immunogenic Vibrio cholerae somatic antigens. Prior studies have shown that the response to vibrios in intact animals appeared to be a true primary response since no pre-existing antibody forming cells were present in non-immunized animals and the antigen induced response, following a lag of two days, resulted thereafter in a rapid stepwise increase in the number of specific PFCs, reaching a peak at 12 to 14 days. Using the Marbrook culture system for antibody formation a readily detected immunocyte response to vibrios was induced with splenocytes from normal non-immunized animals. No background antibody forming cells developed to the organisms without addition of vibrios in vitro. After in vitro immunization with 10(6) bacteria significant numbers of IgM PFCs appeared with a peak response on days 7 to 8. Splenocyte cultures from mice primed earlier with vibrios developed a marked secondary response, with appearance of both IgM and IgG PFCs. Large numbers of both classes of PFCs developed, with peak responses on days 6 to 7, similar to the "primary" response to sheep erythrocytes. However, significant numbers of PFCs to vibrios developed in cultures of vibrio-primed cultures even in the absence of vibrios during the 12 day culture period. The availability of a completely in vitro model system to induce a true primary immune response without the complication of pre-existing background antibody forming cells will be of value for further studies concerning various cellular pathways and interactions during the immune response to small amounts of strongly immunogenic bacterial antigens.

Modulation of the immune response in vitro and in vivo by splenocytes from tumor-bearing mice

Immunosuppression in mice infected with either a leukemia virus (Friend virus) or bearing a rapidly growing transplantable tumor (either a plasmacytoma or mastocytoma) was studied at the level of individual immunocytes to sheep erythrocytes both in vivo and in vitro. Immunization of mice with progressing tumors showed markedly depressed hemolytic antibody plaque responses in the spleen. Furthermore, spleen cell cultures derived from immunodepressed mice with the tumors revealed the continued impairment of antibody formation in vitro. Relatively small numbers of splenocytes from the tumor-bearing mice suppressed larger numbers of normal spleen cells from control mice. Immunosuppression in all three tumor systems could be related to subcellular factors in that cell-free extracts of the spleens and tumor bearing mice or even ascites fluid or plasma could suppress the normal antibody responsiveness of normal spleen cell cultures. The virus per se or a virus associated factor seemed important in the leukemia virus model but non-virus tumor associated or related substances seemed to be involved in the immunosuppression induced by the plasmacytoma or mastocytoma. Such results support the view that tumor related subcellular factors may be important mediators of immunologic impairment of a host's immune defense mechanism to a neoplasm.