Development and analysis of transgenic mice expressing porcine hematopoietic cytokines: a model for achieving durable porcine hematopoietic chimerism across an extensive xenogeneic barrier

The capacity of mixed hematopoietic chimerism to induce tolerance has not been demonstrated in discordant xenogeneic species combinations because of the difficulty in achieving lasting hematopoietic engraftment. In an effort to create a model of long-lasting disparate xenogeneic hematopoietic chimerism, we have developed transgenic (Tg) mice carrying porcine cytokines. Three lines of Tg mice were generated: one carrying porcine IL-3 and GM-CSF genes only (termed IL/GM) and the remaining two lines carrying in addition, the soluble SCF gene (termed IL/GM/sS) or membrane-bound SCF gene (termed IL/GM/mS). Sera from mice with IL/GM and IL/GM/sS transgenes markedly stimulated the proliferation of swine marrow cells in vitro. However, proliferation of swine marrow cells was not induced in cultures containing IL/GM/mS sera. Consistent with these observations, ELISA assays revealed detectable levels of porcine cytokines in the sera of IL/ GM and IL/GM/sS, but not in sera of IL/GM/mS Tg mice. Marrow stromal cells prepared from all three kinds of Tg mice, but not those from non-Tg littermates, were capable of supporting the growth of porcine hematopoietic cells in vitro. Immunodeficient Tg mice were generated by crossing Tg founders with C.B-17 SCID mice for five generations. All Tg immunodeficient mice showed improved porcine hematopoietic engraftment compared with non-Tg controls. These Tg mice provide a useful model system for studying porcine hematopoietic stem cells, and for evaluating the feasibility of donor-specific tolerance induction by mixed chimerism across highly disparate xenogeneic barriers.

Immunohistologic evaluation of mechanisms mediating hyperacute lung rejection, and the effect of treatment with K76-COOH, FUT-175, and anti-Gal column immunoadsorption

Although most investigators agree that lung dysfunction occurs rapidly in various pig-to-primate hyperacute lung rejection (HALR) models, the basic mechanisms mediating this phenomenon remain in question. Here we describe an immunohistochemical method for assessment of mechanisms driving HALR. Using an established model wherein piglet lungs are perfused ex vivo with human blood, six experimental groups (K76 COOH; FUT-175; K76 with FUT; anti-alpha-Gal column adsorption; column with FUT; and column with K76) and two control groups (unmodified human blood; autologous pig blood) were studied. Each lung was biopsied serially during perfusion, and assessed using an immunohistochemical technique, with vWF staining as an internal control to quantitate binding of human IgM, IgG, C3, C5b-9, properdin, and C1q. The effect of each treatment and subsequent lung perfusion on IgG and IgM anti-alpha-Gal titers(by ELISA) and on pig endothelial cell cytotoxicity were correlated with histologic findings. We found that [1] the classical complement activation pathway was activated, as has been shown for other pig organs in primate or human blood environments [2]; alternative complement pathway activation is also seen, which has not been described for other organs in pig-to-primate models, but only in the context of classical pathway activation; and [3] anti-Gal column absorption, pharmacologic inhibition of complement, or combination therapy each was associated with histologic evidence of partial protection, consistent with what would be predicted for each intervention. Further, immunohistologic differences correlated with physiologic outcomes [8] and with antibody assay results, and revealed that treatments used were incompletely effective. Our data suggest that more complete inhibition of antibody- and complement-driven pathways than was achieved in these experiments will be necessary to prevent the antibody and complement-mediated facets of hyperacute lung rejection. This immunohistologic technique may also help us identify additional pathogenic mechanisms important to eventual clinical application of pig-to-human lung xenografts.

Different responses of human anti-HLA and anti-alphagal antibody to long-term intravenous immunoglobulin therapy

Concentrated human immunoglobulin (IVIG) has been administered intravenously in the treatment of autoimmune disorders and to reduce anti-HLA antibodies in highly sensitized patients awaiting organ transplantation. It has also been shown, in experimental animals, to prevent the hyperacute rejection of discordant xenografts, possibly by anticomplement activity. The aim of the present study was to assess the effect of IVIG therapy on both acquired anti-HLA antibodies and natural antigalactose alpha1-3 galactose (alphaGal) antibodies in five patients awaiting heart transplantation. Five patients placed on mechanical circulatory support who had developed high HLA panel-reactive antibodies (PRA) or in whom the percentage of PRA was increasing rapidly were treated weekly with 500 mg/kg IVIG, which contained 1% of anti-alphaGal IgG. Levels of PRA, anti-alphaGal IgG and IgM, and serum cytotoxicity to pig cells were measured before, during, and after therapy. PRA percentages in the five patients were initially 85%, 53%, 23%, 19% and 19% (mean 39%). Mean PRA fell by 66% after 3 months of therapy (to a mean PRA of 14%), and by 96% after 6 months therapy (to a mean PRA of 2%). Anti-alphaGal antibody levels and serum cytotoxicity to pig aortic endothelial cells did not change significantly. These results confirm the effectiveness of IVIG therapy in reducing PRA in HLA highly sensitized patients. It is likely that IVIG does not contain the relevant anti-HLA antibody, resulting in an accelerated catabolism of native alloantibodies. However, as IVIG contains a normal level of anti-alphaGal IgG, catabolism of anti-alphaGal IgG is not modified, as it is being continuously replaced. To achieve a decrease in the anti-alphaGal IgG level it would be necessary to use IVIG depleted of this antibody.

Transfer of swine major histocompatibility complex class II genes into autologous bone marrow cells of baboons for the induction of tolerance across xenogeneic barriers

BACKGROUND

The present study examined the potential role of gene therapy in the induction of tolerance to anti-porcine major histocompatibility complex (SLA) class II-mediated responses after porcine renal or skin xenografts.

METHODS

Baboons were treated with a non-myeloablative or a myeloablative preparative regimen before bone marrow transplantation with autologous bone marrow cells retrovirally transduced to express both SLA class II DR and neomycin phosphotransferase (NeoR) genes, or the NeoR gene alone. Four months or more after bone marrow transplantation, the immunological response to a porcine kidney or skin xenograft was examined. Both the renal and skin xenografts were SLA DR-matched to the transgene, and recipients were conditioned by combinations of complement inhibitors, adsorption of natural antibodies, immunosuppressive therapy, and splenectomy.

RESULTS

Although the long-term presence of the SLA transgene was detected in the peripheral blood and/or bone marrow cells of all baboons, the transcription of the transgene was transient. Autopsy tissues were available from one animal and demonstrated expression of the SLA DR transgene in lymphohematopoietic tissues. After kidney and skin transplantation, xenografts were rejected after 8-22 days. Long-term follow-up of control animals demonstrated that high levels of induced IgG antibodies to new non-alphaGal epitopes developed after organ rejection. In contrast, induced non-alphaGal IgG antibody responses were minimal in the SLA DR-transduced baboons.

CONCLUSIONS

Transfer and expression of xenogeneic class II DR transgenes can be achieved in baboons. This therapy may prevent late T cell-dependent responses to porcine xenografts, which include induced non-alphaGal IgG antibody responses.

Porcine kidney and heart transplantation in baboons undergoing a tolerance induction regimen and antibody adsorption

BACKGROUND

Xenotransplantation would provide a solution to the current shortage of organs for transplantation. Our group has been successful in inducing tolerance in mice and monkey models of allogeneic transplantation. The present study attempts to extend the same tolerance-inducing regimen to a pig-to-baboon organ transplantation model.

METHODS

Nine baboons underwent a conditioning regimen (consisting of nonmyeloablative or myeloablative whole body and thymic irradiation, splenectomy, antithymocyte globulin, pharmacologic immunosuppression and porcine bone marrow transplantation [BMTx]), which has previously been demonstrated to induce donor-specific allograft tolerance in monkeys. In addition, immunoadsorption of anti-alphaGal antibody (Ab) was performed. Four of the nine baboons received pig kidney transplants (KTx), and one also underwent repeat transplantation with an SLA-matched kidney. Two received heterotopic pig heart transplants (HTx). Three baboons underwent conditioning without organ transplantation for long-term studies of natural Ab kinetics.

RESULTS

In the three baboons that received the conditioning regimen without an organ transplant, immunoadsorption reduced Ab by approximately 90%, but recovery of Ab to pretreatment level or higher occurred within 7 days. In contrast, the level of Ab remained low after organ transplant. No Ab to pig antigens other than alphaGal was detected in any baboon before or after BMTx, KTx, or HTx. No graft succumbed to hyperacute rejection. KTx function began to deteriorate within 3-6 days, with oliguria and hematuria progressing to anuria, and the kidneys were excised after 3, 6, 9, 11, and 14 days, respectively. One HTx ceased functioning at 8 days; the second baboon died with a contracting HTx at 15 days. Features of coagulopathy and thrombocytopenia developed in all six transplanted baboons (high D-dimer, prolonged prothrombin time and partial thromboplastin time, and falling fibrinogen) resulting in serious bleeding complications in two baboons, one of which died on day 9. Donor organs showed progressive acute humoral rejection with deposits of IgM, IgG, and complement; a focal mononuclear cellular infiltrate was also observed. The ureter was the earliest structure of the KTx affected by rejection, with progression to necrosis.

CONCLUSIONS

This conditioning regimen prevented hyperacute rejection but was ineffective in preventing the return of Ab, which was associated with the development of acute humoral rejection with features of coagulopathy. No baboon developed anti-pig Ab other than alphaGal Ab. Further modifications of the protocol directed toward suppression of production of Ab are required to successfully induce tolerance to pig organs in baboons.

A phase II study of BTI-322, a monoclonal anti-CD2 antibody, for treatment of steroid-resistant acute graft-versus-host disease

BTI-322, a rat monoclonal IgG2b directed against the CD2 antigen on T cells and natural killer (NK) cells, blocks primary and memory alloantigen proliferative responses in vitro. We have evaluated the pharmacokinetics and safety of BTI-322 during treatment of 20 transplant recipients with steroid-refractory acute graft-versus-host disease (GVHD). Treatment consisted of BTI-322 by intravenous (IV) bolus or 30-minute infusion at approximately 0.1 mg/kg/d for 10 days in addition to continuing high-dose steroids and tacrolimus or cyclosporine. Pharmacokinetic sampling was performed in 10 patients; the t1/2 +/- SE was 9.1 +/- 1.3 hours, the Cmax was 2,549 +/- 291 ng/mL, the Vd was 3.97 +/- 0.95 L, and the Vd/kg was 0. 05 +/- 0.01 L/kg. Ten patients experienced transient dyspnea sometimes accompanied by nausea, vomiting, diarrhea, and tachycardia shortly after the initial bolus dose of drug, but serious drug-related adverse events were not seen during the remainder of the infusions. At the end of treatment (day 11), there were six patients with complete responses and five with a reduction in grade of GVHD for a total response rate of 55% (95% confidence interval [CI], 32% to 77%). Antibodies targeting CD2 may be active in the treatment of acute GVHD, and evaluation of a humanized form of BTI-322 is warranted.

Pharmacologic immunosuppressive therapy and extracorporeal immunoadsorption in the suppression of anti-alphaGal antibody in the baboon

The aim of this study was to deplete baboons of anti-(alpha)galactosyl (alphaGal] antibody and attempt to maintain depletion by pharmacologic immunosuppressive therapy (PI). In 12 experiments, involving nine baboons, repeated extracorporeal immunoadsorption (EIA) was carried out by plasma perfusion through immunoaffinity columns of synthetic alphaGal trisaccharide type 6. Five of the baboons were immunologically naive and four had undergone various procedures at least 6 months previously. All, however, had recovered lymphohematopoietic function and (with one exception) had levels of anti-alphaGal antibody within the normal range. Eleven protocols included continuous i.v. cyclosporine (to maintain whole blood levels of approximately 1,600 ng/ml). In addition, in ten protocols, the baboon received one or more of the following drugs: cyclophosphamide (1-20 mg/kg/day), mycophenolate mofetil (70-700 mg/ kg/day), brequinar sodium (1-12 mg/kg/day), prednisolone (1 mg/kg/day), melphalan (0.15-0.6 mg/kg/day), methylprednisolone (125 mg/day x3), and antilymphocyte globulin (ATG) (50 mg/kg/day x3). EIA was carried out on 1-9 occasions in each study and was temporarily successful in removing all antibody. When no PI was administered, antibody returned close to pre-EIA levels within 48 hr. Cyclosporine alone delayed the rate of antibody return only slightly. While EIA was continuing on a daily or alternate day schedule, antibody levels (both IgM and IgG) were maintained at 20-45% of pre-EIA levels. Once EIA was discontinued but PI maintained, IgM rose to 40-90% and IgG to 30-60% of pre-EIA levels. In vitro testing demonstrated significant cytotoxicity to pig cells at these antibody levels. We conclude that i) EIA utilizing columns of alphaGal trisaccharide is successful in temporarily depleting baboons of anti-alphaGal antibody, but ii) none of the PI regimens tested suppressed antibody production to levels which would be expected to prevent antibody-mediated rejection of pig xenografts. Additional strategies will therefore be required if xenotransplantation is to become a clinical reality.

Anti-Gal(alpha)1-3Gal antibody response to porcine bone marrow in unmodified baboons and baboons conditioned for tolerance induction

BACKGROUND

Mixed lymphohematopoietic chimerism can provide an effective means of inducing longterm immunological tolerance and has been documented in a monkey allograft model. A conditioning regimen including nonmyeloablative or myeloablative irradiation and splenectomy has been used to induce chimerism in a pig-to-primate transplantation model. Since the presence of anti-Gal(alpha)1-3Gal (alphaGal) natural antibodies leads to the hyperacute rejection of pig organs transplanted into primates, extracorporeal immunoaffinity adsorption (EIA) of anti-alphaGal antibodies is also included in the regimen. The effect of the tolerance induction protocol on the anti-alphaGal antibody response has been assessed.

METHODS

Anti-alphaGal antibody was measured after the EIA of plasma through an alphaGal immunoaffinity column in baseline studies involving two unmodified baboons, one splenectomized baboon, and one baboon that received a challenge with porcine bone marrow (BM), and in three groups of baboons (n=2 in each group) that received different conditioning regimens for tolerance induction. Group 1 received a nonmyeloablative conditioning regimen without porcine BM transplantation. Group 2 received nonmyeloablative conditioning with pig BM transplantation and pig cytokine therapy. Group 3 received myeloablative conditioning, an autologous BM transplant (with BM depleted of CD2+ or CD2+/CD20+ cells), and pig BM transplantation.

RESULTS

In the baseline studies, a single EIA of anti-alphaGal antibodies in an unmodified animal initially depleted anti-alphaGal antibody, followed by a mild rebound. Nonmyeloablative conditioning (group 1) in the absence of pig cell exposure reduced the rate of anti-alphaGal antibody return. Pig BM cells markedly stimulated anti-alphaGal antibody production in an unmodified baboon (alphaGal IgM and IgG levels increased 40- and 220-fold, respectively). This response was significantly reduced (to an only 2- to 5.5-fold increase of IgM and IgG) in baboons undergoing nonmyeloablative conditioning (group 2). A myeloablative conditioning regimen (group 3) prevented the antibody response to pig BM, with the reduction in response being greater in the baboon that received autologous BM depleted of both CD2+ and CD20+ cells. No new antibody directed against pig non-aGal antigens was detected in any baboon during the 1 month follow-up period.

CONCLUSIONS

(i) EIA of anti-alphaGal antibody in unmodified baboons results in a transient depletion followed by a mild rebound of antibody; (ii) exposure to pig BM cells results in a substantial increase in anti-alphaGal antibody production; (iii) a nonmyeloablative conditioning regimen reduces the rate of antibody return and (iv) markedly reduces the response to pig BM cells; (v) the anti-alphaGal response is completely suppressed by a myeloablative regimen if CD2+ and CD20+ cells are eliminated from the autologous BM inoculum. Furthermore, (vi) challenge with pig BM cells appears to stimulate only an anti-alphaGal antibody response without the development of other (non-alphaGal) anti-pig antibodies. We conclude that regimens used for T-cell tolerance induction can be beneficial in reducing the anti-alphaGal antibody response to porcine BM.

Depletion of anti-Gal(alpha)1-3Gal antibody in baboons by specific alpha-Gal immunoaffinity columns

Ongoing studies at our center on facilitating transplantation of discordant xenogeneic organs are focused on tolerance induction. To abrogate hyperacute rejection, we have used adsorption methods to eliminate natural anti-Gal(alpha)1-3Gal (alphaGal) antibodies from the circulation of baboons. We have analyzed data concerning antibody removal in baboons that were 1) immunologically naive, 2) receiving conventional pharmacologic immunosuppressive therapy (IS), and 3) treated with a conditioning regimen for tolerance induction. We compared the efficiency of removing alphaGal antibody 1) by perfusion of whole blood through an alphaGal affinity column (CP; n=5) with 2) perfusion of plasma (separated from cellular components by apheresis) through an alphaGal column (CPA; n=39). Our studies demonstrate that 1) CP and CPA are equally effective in removing anti-alphaGal antibody, 2) CPA is the method of choice if multiple adsorptions are required, 3) CPA in naive animals transiently affects levels of total IgG and IgM, 4) four CPAs repeated at 2-4 day intervals in association with heavy IS reduce the pool of anti-alphaGal antibody and total Ig, and 5) splenectomy and/or IS delay the return of anti-alphaGal antibody.

Removal of anti-porcine natural antibodies from human and nonhuman primate plasma in vitro and in vivo by a Galalpha1-3Galbeta1-4betaGlc-X immunoaffinity column

BACKGROUND

Natural antibodies (NAbs) against a terminal alpha1-3 galactosyl (alphaGal) epitope have been identified as the major human anti-pig NAbs.

METHODS AND RESULTS

We used two synthetic alphaGal trisaccharides--type 6 (alphaGal6) and type 2(alphaGal2)--linked to an inert matrix to remove NAbs from human plasma in vitro. Flow cytometry indicated that an average of 85% of the NAb binding activity was depleted by adsorption with alphaGal6. By measuring the binding of NAbs to pig peripheral blood mononuclear cells and bone marrow cells, we demonstrated that alphaGal6 was more effective than alphaGal2 in removing NAbs, and the combination of alphaGal6 + alphaGal2 did not further increase removal of NAbs. The specificity of the removal of NAbs (IgM and IgG) reactive with the alphaGal epitope by alphaGal6 matrix was shown by enzyme-linked immunosorbent assay. In vivo studies in nonhuman primates compared plasma perfusion through a alphaGal6 immunoaffinity column with hemoperfusion through a pig liver for changes in blood pressure, hematocrit, platelets, and NAb adsorption.

CONCLUSIONS

Both methods reduced the level of anti-pig IgM and IgG xenoreactive antibodies to nearly background, but column perfusion caused less hypotension and reduction in platelets than liver perfusion. Four pig kidneys transplanted into monkeys after column perfusion did not undergo hyperacute rejection, remaining functional for 2-10 days, with a mean functional period of 7 days, demonstrating that a pig kidney can support renal function in a primate.

Modification of monoclonal antibody carbohydrates by oxidation, conjugation, or deoxymannojirimycin does not interfere with antibody effector functions

Site-specific attachment of metal chelators or cytotoxic agents to the carbohydrate region of monoclonal antibodies results in clinically useful immunoconjugates [Doerr et al. (1991) Ann Surg 214: 118, Wynant et al. (1991) Prostate 18: 229]. Since the capacity of monoclonal antibodies (mAb) to mediate tumor cell lysis via antibody-dependent cellular cytotoxicity (ADCC) or complement-dependent cytotoxicity (CDC) may accentuate the therapeutic effectiveness of immunoconjugates, we determined whether site-specific modification of mAb carbohydrates interfered with these functions. The chemical modifications examined consisted of periodate oxidation and subsequent conjugation to either a peptide linker/chelator (GYK-DTPA) or a cytotoxic drug (doxorubicin adipic dihydrazide). mAb-associated carbohydrates were also modified metabolically by incubating hybridoma cells in the presence of a glucosidase inhibitor deoxymannojirimycin to produce high-mannose antibody. All four forms (unaltered, oxidized, conjugated and high-mannose) of murine mAb OVB-3 mediated tumor cell lysis via CDC. Similarly, equivalent ADCC was observed with native and conjugated forms of mAb OVB-3 and EGFR.1. ADCC was achieved with different murine effector cells such as naive (NS), poly (I*C)- and lipopolysaccharide-stimulated (SS) spleen cells, or Corynebacterium-parvum-elicited peritoneal cells (PEC). All murine effector cell types mediated tumor cell lysis but differed in potency such that PEC > SS > NS. Excellent ADCC activity was also demonstrable by human peripheral blood mononuclear cells with OVB-3-GYK-DTPA and high-mannose OVB-3 mAb. ADCC activity was detectable in vivo: both native and conjugated OVB-3 inhibited growth of OVCAR-3 xenografts in nude mice primed with C. parvum. In conclusion, modification of mAb carbohydrates did not compromise their in vivo or in vitro biological functions. Therefore, combination therapy using immunomodulators to enhance the effector functions of site-specific immunoconjugates could be seriously contemplated.

Elimination of cycling CD4+ suppressor T cells with an anti-mitotic drug releases non-cycling CD8+ T cells to cause regression of an advanced lymphoma

This paper describes a model of successful immunotherapy of advanced lymphoma based on the selective elimination of cycling tumour-induced suppressor T cells. It shows that a single injection of the anti-mitotic drug, vinblasting (Vb), results in complete regression of a large L5178Y lymphoma and its metastases, but not if it is growing in an immunocompetent host. Vb-induced, immunologically mediated tumour regression was dependent on the anti-tumour function of CD8+ T cells, because regression was prevented by depleting the host of this subset of T cells 24 hr after Vb was given. Regression was also prevented by infusing the host with Vb-sensitive, CD4+ T cells from a tumour-bearing donor. These and other results are in keeping with the interpretation that Vb-induced regression of the L5178Y lymphoma depends on the ability of the drug to eliminate CD4+ suppressor T cells that are replicating, and to spare non-replicating CD8+ effector cells. It is suggested that at an advanced stage of growth of the L5178Y lymphoma the host possesses an acquired population of antigen-primed CD8+ effector T cells that are unable to become activated in response to abundant tumour antigen because of the dominant influence of CD4+ suppressor cells. Activation of these CD8+ T cells was indicated by the finding that they were rapidly converted from being cyclophosphamide (Cy) resistant to being highly Cy sensitive within 48 hr of giving Vb.

Radiosensitive barrier to T-cell-mediated adoptive immunotherapy of established tumors

This study shows that it is not possible to cause regression of the immunogenic SA-1 sarcoma by adoptive immunotherapy with tumor-sensitized T-cells, unless the tumor-bearing recipient is exposed to a sublethal dose of gamma-irradiation to remove a barrier that prevents adoptive immunity from being expressed. This barrier to adoptive immunotherapy was found to be regenerated between 2 and 4 weeks following irradiation, and its regeneration was associated with general repopulation of host T-cells. However, it was not regenerated in the absence of the thymus, thus showing that it is T-cell dependent. Evidence that it is caused by the presence of CD4+ suppressor T-cells was shown by the finding that it can be removed by depleting mice of CD4+ T-cells with anti-L3T4 monoclonal antibodies, but not by depleting them of CD8+ T-cells with anti-Lyt-2 monoclonal antibodies. Again, the barrier could be restored to irradiated recipients by infusing them with CD4+ T-cells, but not with CD8+ T-cells, from tumor-bearing donors. The barrier to adoptive immunotherapy was found to be tumor induced and to be paradoxically generated in concert with host concomitant immunity.

Elimination of CD4+ suppressor T cells from susceptible BALB/c mice releases CD8+ T lymphocytes to mediate protective immunity against Leishmania

This study examined the capacity of BALB/c mice that had been depleted of T cell subpopulations to generate a protective immune response to Leishmania major. Thymectomized mice were depleted of either L3T4+ (CD4+) T lymphocytes, Ly2+ (CD8+) T lymphocytes, or both, by treatment with appropriate mAbs. It was found that susceptible mice were rendered resistant to Leishmania by an intravenous infusion of anti-L3T4 mAb. These mice generated an immune response that destroyed the parasite in the primary lesion and in visceral metastatic foci. CD4+ cell-depleted mice also acquired a capacity to mount a sustained delayed-type hypersensitivity (DTH) response to parasite antigens, indicating that DTH, per se, is not a disease-promoting mechanism in the susceptible murine host as has been suggested. Depleting BALB/c mice of CD8+, as well as CD4+ T cells, left them highly susceptible to Leishmania infection, thereby indicating that CD8+ lymphocytes are key protective cells. Our results can be interpreted as showing that the susceptibility of BALB/c mice is due to the generation of CD4+ cells that suppress either the generation or expression of CD8+ T cell-mediated antiLeishmania immunity.

Cyclophosphamide-induced immunologically mediated regression of a cyclophosphamide-resistant murine tumor: a consequence of eliminating precursor L3T4+ suppressor T-cells

It was shown that it is possible to use cyclophosphamide (Cy) to cause immunologically mediated regression of the immunogenic, Cy-resistant L5178Y lymphoma in syngeneic and semisyngeneic mice. In order to cause tumor regression it was necessary to give Cy shortly before or shortly after tumor implantation. However, regardless of whether Cy was given before or after tumor implantation, tumor regression did not commence until 10 days of progressive tumor growth, by which time the tumor was 1 cm in diameter. Tumor regression was associated with the presence in the spleen of an increased number of Lyt-2+ T-cells capable of passively transferring immunity to tumor-bearing recipients. This augmented level of immunity was sustained throughout the period of tumor regression. In contrast, a lower level of concomitant immunity generated by control tumor bearers decayed after Day 12 of tumor growth. Because the therapeutic effect of Cy could be inhibited by passive transfer of L3T4+ T-cells from normal donor mice it is apparent that the therapeutic effect of Cy is based on its ability to preferentially destroy L3T4+ suppressor T-cells. These putative precursor suppressor T-cells were regenerated 4 days after being destroyed by Cy. Taken together the results represent a striking example of the negative regulatory influence of suppressor T-cells on the immune response to an immunogenic tumor.

Immunologically mediated regression of a murine lymphoma after treatment with anti-L3T4 antibody. A consequence of removing L3T4+ suppressor T cells from a host generating predominantly Lyt-2+ T cell-mediated immunity

This study shows that intravenous injection of 1 mg of anti-L3T4 mAb (GK1.5) into thymectomized mice bearing the syngeneic L5178Y lymphoma results, after a delay of 2-3 d, in complete regression of this tumor and in long-term host survival. A flow cytofluorometric examination of the spleen cells of mAb-treated mice revealed that antibody treatment resulted in the elimination of greater than 98% of L3T4+ T cells, but had no effect on the Lyt-2+ T cells subset. Tumor regression was immunologically mediated, because L5178Y lymphoma cells were shown to be L3T4-, and regression of the tumor failed to occur in mice that had been lethally irradiated before anti-L3T4 mAb was given. Tumor regression was mediated by tumor-sensitized Lyt2+ T cells, as evidenced by the finding that treatment of tumor-bearing mice with anti-Lyt-2 mAb alone, or in combination with anti-L3T4 mAb, resulted in enhancement of tumor growth and a significant decrease in host survival time. Moreover, the spleens of mice whose tumors were undergoing regression in response to anti-L3T4 mAb treatment contained Lyt-2+ T cells capable, on passive transfer, of causing regression of a tumor in recipient mice. These results can be interpreted as showing that removal of tumor-induced L3T4+ suppressor T cells results in the release of Lyt-2+ effector T cells from suppression, and consequently in the generation of enough Lyt-2+ T cell-mediated immunity to cause tumor regression. This can only be achieved, however, if immunity to the tumor is mediated exclusively by Lyt-2+ T cells, as is the case for the L5178Y lymphoma. In the case of the P815 mastocytoma, treatment with anti-L3T4 mAb was without a therapeutic effect, and this was in keeping with the finding that immunity to this tumor is mediated by L3T4+, as well by Lyt-2+ T cells.

Cyclophosphamide (Cy)-facilitated adoptive immunotherapy of a Cy-resistant tumour. Evidence that Cy permits the expression of adoptive T-cell mediated immunity by removing suppressor T cells rather than by reducing tumour burden

A cyclophosphamide (Cy)-resistant immunogenic tumour, the L5178Y lymphoma, was used to demonstrate that Cy-treatment of a host bearing this tumour enables passively transferred tumour-sensitized T cells to cause complete tumour regression without any need for Cy to cause a reduction in tumour burden. It was shown that whereas infusion of tumour-sensitized T cells from immune donors had very little effect on growth of the tumour, and whereas treatment with 150 mg/kg of Cy caused appreciable enhancement of tumour growth, combination therapy with Cy plus immune T cells caused complete tumour regression and resulted in long-term survival. Evidence that Cy treatment facilitated the expression of adoptive immunity against the L5178Y lymphoma by eliminating tumour-induced suppressor T cells consisted of the demonstration that tumour regression caused by combination treatment with Cy and immune T cells could be inhibited by infusing the recipient with Cy-sensitive, L3T4+ T cells from tumour-bearing but not from normal donors.

Sublethal, whole-body ionizing irradiation can be tumor promotive or tumor destructive depending on the stage of development of underlying antitumor immunity

It was shown that sublethal (500 rads), whole-body gamma-irradiation of mice bearing an established i.d. immunogenic tumor can result, after several days delay, in complete tumor regression and long-term survival, but only if radiation is given after the tumor is established and growing progressively. Exposing mice to the same dose of radiation several hours after tumor cells were implanted resulted, in contrast, in enhanced growth of the primary tumor and in earlier death from systemic disease. Irradiation-induced tumor regression failed to occur in mice that were incapable of generating antitumor immunity, because of having been made T cell deficient by thymectomy and irradiation. Again, irradiation-induced tumor regression could be blocked by infusion of spleen cells from donor mice bearing a well-established tumor. These and previously published results support the view that sublethal, whole-body ionizing irradiation causes tumor regression by preferentially destroying radiosensitive suppressor T cells, thereby enabling the host to generate a therapeutic level of concomitant immunity. It is suggested that the preferential destruction of suppressor cells by irradiation depends on the acquisition, during immunologic induction, of radioresistance by antigen-activated effector T cells, and that this is the reason irradiation causes regression only of established tumors. Not all tumors tested were immunogenic enough to undergo regression in response to gamma-irradiation.