T cell leukaemia in the rat: the pathophysiology

IL-12, IL-15, and IL-18 pre-activated NK cells target resistant T cell acute lymphoblastic leukemia and delay leukemia development in vivo

NK cells have shown promise in therapy of hematological cancers, in particular against acute myeloid leukemia. In contrast, the more NK cell-resistant acute lymphoblastic leukemia (ALL) is difficult to treat with NK-cell-based therapies, and we hypothesized that pre-activation of NK cells could overcome this resistance. We show in pediatric and adult patients with T-cell ALL (T-ALL) perturbed NK cell effector functions at diagnosis. Using an in vivo rat model for T-ALL, Roser leukemia (RL), suppressed NK cell effector functions were observed. NK cells from T-ALL patients had reduced expression of the activating receptors NKp46 and DNAM-1, but not NKG2D. In contrast to T-ALL patients, NKG2D but not NKp46 was downregulated on NK cells during rat RL. Decreased frequencies of terminally differentiated NKG2A⁺CD57^-CD56^dim NK cells in human T-ALL was paralleled in the rat by reduced frequencies of bone marrow NK cells expressing the maturation marker CD11b, possibly indicating impairment of differentiation during leukemia. RL was highly resistant to autologous NK cells, but this resistance was overcome upon pre-activation of NK cells with IL-12, IL-15, and IL-18, with concomitant upregulation of activation markers and activating receptors. Importantly, adoptive transfers of IL-12, IL-15, and IL-18 pre-activated NK cells significantly slowed progression of RL in vivo. The data thus shows that T-ALL blasts normally resistant to NK cells may be targeted by cytokine pre-activated autologous NK cells, and this approach could have potential implications for immunotherapeutic protocols using NK cells to more efficiently target leukemia.

Decontamination of leukemic cells and enrichment of germ cells from testicular samples from rats with Roser’s T-cell leukemia by flow cytometric sorting

Testicular germ cell transplantation is a novel strategy for preservation of fertility in prepubertal cancer patients, but the risk of reseeding tumor cells into cured patients presently limits clinical application of this approach. To date, no systematic evaluation of the limitations of surface marker-based decontamination of testicular samples with acute lymphoblastic leukemia has been performed. Here, surface markers for leukemic (CD4 and major histocompatibility complex class I) and germ cells (epithelia cell adhesion molecule) in testicular samples infiltrated with Roser’s T-cell leukemia were identified. These markers were then used to delete leukemic cells and/or select for germ cells by flow cytometry (FACS). The resulting cell populations were analyzed by FACS, immunocytochemistry, or evaluation of leukemia transmission in syngeneic piebald variegated rats. Simple positive selection of germ cells or deletion of leukemic cells using specific surface markers was unable to effectively decontaminate testicular samples. The poor specificity of spermatogonial surface markers and aggregation of germ and leukemic cells limited the positive selection of germ cells, while immunophenotypic variation among lymphoblastic leukemia cells prevented adequate deletion of leukemic cells. Enzymatic treatment to disperse the testicular cells and feature of the intratesticular environment contributed to this immunophenotypic variation. Only germ cell selection in combination with leukemic cell deletion prevented leukemia transmission in association with intratesticular injection of the sorted cells. However, with such combined sorting, only 0.23% of the original testicular cells were recovered. With presently available techniques, flow cytometric purification of germ cells from a leukemic donor is not sufficiently effective or safe for clinical use.

A bioactively modified fatty acid improves survival and impairs metastasis in preclinical models of acute leukemia

PURPOSE

Polyunsaturated fatty acids (PUFA) and the sulfur-substituted fatty acid tetradecylthioacetic acid (TTA) inhibit proliferation and induce apoptosis in lymphoma and leukemic cell lines, but it is unknown if they can modify leukemogenesis in the intact organism.

EXPERIMENTAL DESIGN

We now examined the effects of PUFA and TTA in rats transplanted with either acute promyelocytic leukemia or acute T-cell leukemia. The rats were randomized to isoenergetic diets containing either lard (control), omega3 (n-3) PUFA, or TTA.

RESULTS

Whereas TTA prolonged survival (P < 0.05) in both types of rat leukemia, n-3 PUFA had no significant effect compared with controls. Only TTA inhibited (P < 0.05) leukemic infiltration in the bone marrow and spleen, probably due to apoptosis of the leukemic cells. Plasma metalloproteinase activity, a marker of metastatic activity, was significantly reduced in TTA-fed rats only.

CONCLUSIONS

Dietary intake of TTA, but not of n-3 PUFA, in rats with acute leukemia, prolonged their survival. TTA intake was also associated with reduced leukemic cell burden as well as diminished extramedullar dissemination. TTA represents a modified fatty acid that exerts unique effects on malignant hematopoietic cells, and the present study indicates that TTA may have a therapeutic potential in patients with acute leukemias.

Phenotype and natural killer cell sensitivity of a radiation-induced acute T-cell leukaemia (Roser leukaemia) in PVG rats

A radiation-induced T-cell leukaemia [Roser leukaemia (RL)] in the rat was conditioned for growth in vitro by repeated in vivo-in vitro passages. This in vitro cell line, termed RL-T, maintained its leukaemia-inducing property when transferred to syngeneic PVG rats. It expresses several T-cell markers and the T-cell alpha/beta receptor-CD3 complex. RL-T, furthermore, expresses major histocompatibility complex (MHC) I antigens, both classical (RT1.A) and nonclassical (RT1.C), which makes it susceptible to killing by alloreactive natural killer cells in vitro.

Involvement of prostaglandins in cachexia induced by T-cell leukemia in the rat

We have previously demonstrated that experimentally induced T-cell leukemia in the rat results in a rapid and severe cachexia. This weight loss is largely due to a reduction in food intake, but is also accompanied by inappropriately high rates of energy expenditure. Increases in resting oxygen consumption (VO2) of 25% to 35% above the levels of pair-fed animals were observed over the period of weight loss. The present study investigated the possible involvement of prostaglandins in the cachexia induced by T-cell leukemia in the rat. Acute systemic injection of the cyclo-oxygenase inhibitors (indomethacin 1 mg/kg or flurbiprofen 1 mg/kg intraperitoneally [IP]) significantly reduced (by 14% and 10%, respectively) the increase in metabolic rate and also reversed the elevated body temperature of leukemic animals. Intracerebroventricular (ICV) injection of indomethacin (0.2 mg/kg) had only modest effects on the increase in temperature or hypermetabolism of leukemic animals. Long-term daily injection of indomethacin or flurbiprofen (1 mg/kg/d IP) had no significant effect on food intake or body weight of leukemic animals, and neither treatment significantly affected disease status. Indomethacin significantly reduced the decline in epididymal fat pad weight of leukemic animals. These data indicate that prostaglandins, produced peripherally, are involved in the acute hypermetabolism associated with T-cell leukemia, but have little or no effect on the hypophagia or body weight loss of leukemic rats.

Mechanisms of cachexia induced by T-cell leukemia in the rat

Body wasting (cachexia) is a common feature of cancer and a major cause of morbidity and mortality. The mechanisms underlying cachexia are largely unknown, and studies in experimental animals have focused mainly on solid tumors. Therefore, the objective of the present study was to quantify and investigate cachexia in experimentally induced T-cell leukemia in the rat. Induction of leukemia by serial passage (injection of cervical lymph node suspension) resulted in a rapid increase in white blood cell (WBC count, hypertrophy of the spleen (by day 11), and severe morbidity within 17 to 18 days. Body weight gain and food intake declined steadily in leukemic animals from day 12, although weight loss was significantly greater in pair-fed, nonleukemic animals. However, leukemic rats had a lower body fat content and higher water content than pair-fed animals on day 18, so the measurement of body weight significantly underestimated the severity of cachexia. Resting oxygen consumption (VO2), measured during the light phase, declined in pair-fed animals from day 13, but was elevated in leukemic rats on days 12 to 18 by 25% (P < .05, one-way ANOVA) compared with pair-fed rats and by 7% (P < .05, one-way ANOVA) relative to free-feeding controls. Hypermetabolism was associated with an increase in brown adipose tissue (BAT) activity (74% and 89%, respectively, P < .05, one-way ANOVA) in leukemic rats compared with control and pair-fed groups. Effects of leukemia on VO2 and BAT were prevented by administration of the adrenergic antagonist, propranolol. These results indicate that T-cell leukemia in the rat results in rapid and severe cachexia, which is largely due to marked hypophagia, but is also accompanied by inappropriately high rates of energy expenditure that are mediated by sympathetic activation of BAT thermogenesis.

Effects of testicular cytokines on proliferation of rat T-leukaemic lymphoblasts in vitro

A rat acute lymphoblastic leukaemia (ALL) model was used to study the mechanisms involved in the tendency to testicular relapse of ALL in boys. Previous studies have indicated that the infiltration and growth of leukaemic lymphoblasts in the testis are influenced by the same endocrine and paracrine control systems that regulate normal testicular function. In the present study the effects of aqueous extracts of scrotal, abdominal and estrogen-treated postpubertal rat testes on rat-leukaemic lymphoblast proliferation were evaluated. The effects of recombinant cytokines analogous to those observed in the testis on leukaemic cell DNA-synthesis were also evaluated since changes in the levels of these factors have been observed in association with cryptorchidism and low levels of gonadotropins. Transforming growth factor-beta 1 (TGF-beta1), significantly inhibited the proliferation of leukaemic rat lymphoblasts after 24 h of culture, whereas TGF-beta 2, interleukin-1 alpha (IL-1 alpha), IL-1 beta, IL-6 or combinations of them were inactive. Extracts of estrogen-treated testes and abdominal testes of unilaterally cryptorchid animals inhibited leukaemic T-cell proliferation significantly more than extracts of normal testes. The inhibitory activity in abdominal testes could be neutralized by anti-TGF-beta 1 antibodies. These results suggest that testicular TGF-beta 1 may influence growth of leukaemic lymphoblasts in the testis but also that other as yet unknown, testicular factors are involved in the regulation of leukaemic cell function in the testis.

Spread of malignant lymphoid cells into rat central nervous system with intact and disrupted blood-brain barrier

The pathways of spread of malignant lymphoid cells into the central nervous system (CNS) were studied using a T lymphoblastic leukaemia/lymphoma model of inbred PVG rats. The effects of intraperitoneal, intracarotid, intravenous, intrathecal and intracerebral routes of transplantation were analysed, and the significance of the blood-brain barrier (BBB) in preventing neoplastic cell invasion was studied by disrupting the BBB with focal cold injury. Extraneurally transplanted cells appeared first in the dura and subarachnoid space. From the latter they spread further into the perivascular space of penetrating cortical vessels. Parenchymal tumour cell foci were only seen in terminally ill rats, usually associated with damage to the vessel wall. Intrathecal transplantation did not accelerate the progression of the disease. Intracerebrally transplanted cells readily produced parenchymal infiltrates with diffuse invasion into the white matter, perivascular spreading into the cortex, and contralateral extension along the corpus callosum. Parenchymal invasion did not occur immediately after disruption of the BBB, but in the chronic phase neoplastic cells infiltrated the injured area. In conclusion, the model closely resembles human CNS leukaemia. Malignant cells appeared to enter the CNS through the deficient BBB of the subarachnoid vessels, whereas the BBB of the intracerebral vessels and perivascular glia limitans were very resistant to leukaemic cell invasion. This underlines the difference between the subarachnoid and perivascular v. intraparenchymal compartments. Preceding BBB damage may predispose to brain metastases. The parenchymal dissemination of malignant cells was similar to that in primary CNS lymphoma and it followed the same spreading pathways as the extracellular fluid.

A rodent model for testicular involvement in acute lymphoblastic leukaemia

The testis is the third common site of relapse after primary treatment of childhood acute lymphoblastic leukaemia, but in adults relatively few testicular relapses of acute lymphoblastic leukaemia have been reported. In the present investigation the differences in the behaviour of leukaemia in immature and mature rat testis and the interactions of testicular and leukaemic cells were studied. Intraperitoneal injection of rat T-leukaemic cells to sexually immature animals induced testicular infiltrations in 100% of animals in 17 days. The infiltrations were small and located perivascularly in the interstitial tissue. Intraperitoneal injection of T-leukaemic cells to sexually mature animals induced testicular infiltrates in 42% of the animals. Leukaemic cells injected directly to the lymph sinusoids of sexually immature and mature testis proliferated rapidly causing testicular enlargement. The M(r) > 5 K fraction of extracts of 50 days old normal rat testes inhibited 3H-TdR incorporation of both normal and leukaemic ConA-stimulated rat lymphoblasts significantly. The same fraction of extracts of testes of 25 days old rats did not have any effect on 3H-TdR incorporation. The normally occurring pubertal increase in the lymphocyte inhibitory effect of the M(r) > 5 K fraction of testis extracts on 3H-TdR incorporation of PBL was prevented following either intraperitoneal or intratesticular injection of rat leukaemic lymphoblasts administered at the age of 25 days. The present observations suggest that physiological pubertal changes in the permeability of vascular endothelium and immunosuppressive effect of the testis may be important explanatory factors for the smaller number of testicular relapses in men compared to boys seen after treatment of ALL.

Allospecific recognition of hemic cells in vitro by natural killer cells from athymic rats: evidence that allodeterminants coded for by single major histocompatibility complex haplotypes are recognized

We have previously shown that large granular lymphocyte (LGL)-enriched cell populations have the capacity to spontaneously recognize and kill allogeneic small lymphocytes and bone marrow cells (BMC) in vitro in certain strain combinations of rats. Here, we have studied the alloreactivity of natural killer (NK) cells from PVG nude (RT1c) rats against a panel of major histocompatibility complex (MHC) incompatible hemic cells. Both lymphocytes and BMC from the AO (RT1u), DA (RT1a), BN (RT1n) as well as the MHC-congenic PVG-RT1u (RT1u) rat strains were efficiently killed in vitro, whereas cells from syngeneic PVG rats were spared. The structures recognized on lymphocytes and BMC were probably similar since the two cell populations inhibited each other in cross-competition experiments. A number of features aligned the alloreactive effector cells with NK cells and not T cells. (a) Only about 5% of the effector cells from nude spleens expressed the T cell antigens CD3, CD5 or T cell receptor (TcR) alpha/beta whereas greater than 50% of the cells expressed markers present on NK cells (CD2, CD8, OX52 and the rat NK cell-specific marker NKR-P1 recognized by the monoclonal antibody 3.2.3). (b) The alloreactive cells were granular since pretreatment of nude spleen cells with the lysosomotropic agent L-leucine methyl ester which eliminated LGL, simultaneously abolished the cytolysis of both allogeneic lymphocytes and YAC-1 tumor cells. (c) Nude spleen cells stimulated with human recombinant interleukin 2 for 1 week in vitro generated large granular proliferating cells which were CD3-, CD5-, TcR alpha/beta-, but greater than 95% 3.2.3+. These cells efficiently killed allogeneic hemic cells from the same rat strains as did freshly isolated effector cells. (d) The cytolysis of allogeneic hemic cells could effectively be inhibited with unlabelled NK-sensitive (YAC-1 and K-562), but not NK-resistant (Roser leukemia) tumor cells. Cross-competition studies showed that PVG nude NK cells discriminated between AO, BN and DA BMC, suggesting that different alloantigens were positively recognized by subsets of NK cells. The mode of inheritance of the allodeterminant specifically recognized on AO BMC was investigated in crosses and backcrosses between AO and BN or DA rats. A gene dosage effect was observed in that this determinant was expressed at a slightly reduced level in F1 hybrids.(ABSTRACT TRUNCATED AT 250 WORDS)

Labelling of tumour cells with a biotinylated inhibitor of a cell surface protease

Our objective has been to prepare a biotinylated affinity probe for the active centre of a protease associated with the surface of tumour cells. We employed three model systems in which easily recognisable tumour cells containing the active protease were used as targets for the biotinylated affinity probe. These were: squamous cell carcinoma, leukaemia cells in muscle and outgrowths of prostate carcinoma cells grown in three dimensional collagen gels. The presence of the bound biotinylated affinity probe was demonstrated by its ability to bind Texas-red labelled streptavidin with the results that the tumour cells exhibited red fluorescence. This binding was shown to be competitive with 9-amino acridine, a compound known to bind to the active centre of the target protease. This technique depends upon the affinity of the active centre of an enzyme for a competitive inhibitor and therefore should be applicable to other enzyme systems employing suitable ligands for their active centres.

Competitive inhibition of a tumour cell surface protease. A rapid technique for in vitro testing of selective targeting systems

The active centre of a protease on the surface of tumour cells can be located by its affinity for an active site-directed inhibitor, 9-amino acridine. Cells which have uninhibited proteases, bind 9-amino acridine and fluoresce in resin sections. The leukaemic rat was used as a model system to provide tumour cells in a well defined location. Drugs when coupled to a ligand (directed to the active centre of the protease) compete for this binding site with 9-amino acridine. Thus, competitive inhibition of the tumour cell surface protease provides a rapid technique for demonstrating the delivery of liganded molecules to the surface of tumour cells in vitro.

Influence of cyclosporin A and alpha-difluoromethylornithine, an inhibitor of polyamine biosynthesis, on two rodent T-cell cancers in vivo

We have examined the influence of cyclosporin A (CsA), administered together with the polyamine antimetabolite, alpha-difluoromethylornithine (DFMO), on growth of the Roser acute T-cell leukaemia in PVG rats and on growth of the EL4 lymphoma in C57BL/6 mice. CsA or DFMO alone, administered from the time of tumour injection, markedly reduced numbers of circulating lymphoblasts in leukaemic rats, although survival was prolonged only in those animals given DFMO. Drug combination further reduced blood-borne tumour cells, but had no additional effects on tumour growth within organs or on host survival, compared to that achieved with DFMO treatment alone. Neither CsA nor DFMO, administered from the time of tumour-cell injection, nor both drugs in combination, affected peritoneal growth of the EL4 lymphoma or organ infiltration. Host survival was prolonged by DFMO. As anticipated, DFMO inhibited polyamine synthesis in vivo, but the observed anti-tumour effect of CsA was not accompanied by an alteration in polyamine biosynthesis. By reducing polyamine synthesis, however, DFMO may enhance the vulnerability of those malignant T cells which are susceptible to the as yet unexplained selective inhibitory action of CsA in vivo.

Inhibition of trypsin-like enzymes on cells with rhodamine-aprotinin

Aprotinin, a polypeptide inhibitor of trypsin-like enzymes, has been labelled with rhodamine. Rhodamine-aprotinin inhibits trypsin in free solution in an identical manner to aprotinin. Rhodamine-aprotinin binds to trypsin-like enzymes on cells in formaldehyde fixed wax embedded sections. This technique has been used to locate cells possessing trypsin-like enzymes by means of fluorescent microscopy. In the present study we have used this technique to locate tumour cells.

Influence of cyclosporin A on growth of an acute T-cell leukaemia in PVG rats

Our objective was to examine the effect of cyclosporin A (CsA; 25 or 12.5 mg/kg) on growth of an acute (Roser) T-cell leukaemia in male PVG rats. The leukaemic blasts were shown (by immunocytochemical analysis) to have a mature, T-helper-cell phenotype, i.e., OX-19 (CD5) +/- , W3/25 (CD4)+, OX44+, MHC-class I+, OX-26+, corresponding to a population comprising 5% of normal rat medullary thymocytes. Animals received 20 X 10(3) viable tumour cells intramuscularly (day 0) and were given either CsA (25 or 12.5 mg/kg) or drug vehicle by gavage from day 0 or day 14, by which latter time leukaemic blasts normally appeared in the circulation. Administration of the higher dose of CsA from day 0 or day 14 significantly delayed the appearance of leukaemic cells in the peripheral circulation, whereas treatment with 12.5 mg/kg was without significant effect. CsA whole blood levels on day 17 were twice as high in leukaemic rats as in normal controls. Leukaemic infiltration of the spleen and the liver was reduced on day 17 after 25 mg/kg CsA, but no such effect was observed in lymph nodes or kidneys. A heterogeneous, host "reactive" cell population, which developed in response to the leukaemia, was inhibited by CsA, indicating that the effect of the drug was probably not mediated by host defence mechanisms. In CsA-treated leukaemic animals, there was biochemical evidence of synergistic impairment of glomerular and tubular function.

Location of T-cell leukaemia cells in a model rat system by means of a fluorescent probe

Fluorescence probes for the active centre of an enzyme associated with tumour cells have been used to locate leukaemia cells in a model rat system. These fluorescent techniques are inexpensive and rapid to carry out. The leukaemic cells can be located by fluorescence microscopy in frozen sections, wax embedded sections and resin embedded sections. The technique is illustrated with reference to sections of leukaemic rat kidney, epididymis and testis. These studies confirm earlier histological findings employing conventional staining techniques and have the advantage that individual leukaemia cells can be detected in leukaemic animals undergoing drug therapy. The evidence suggests that these techniques will be of value in further studies of the design of drugs directed to leukaemia cells.

Fluorescent inhibitors of a cell surface protease used to locate leukaemia cells in kidney sections

Guanidinobenzoatase is a trypsin-like protease on the surface of cells capable of migration, for example leukaemia cells. We have used a number of fluorescent probes that are competitive inhibitors of guanidinobenzoatase to locate leukaemia cells in resin sections of kidney tissue obtained from leukaemic rats. We have demonstrated how this competitive inhibition system can be used to direct desired molecules (such as cytotoxic drugs) to these cells and to monitor the arrival of such compounds at the active site of guanidinobenzoatase. The principles developed in this study could equally well be applied to other enzymes on other cells provided suitable competitive inhibitors were designed. The presence of an enzyme on the surface of a cell can be used to direct molecules to that cell provided that these molecules contain a functional group that acts as an inhibitor for the chosen enzyme.

The effect of inhibition of mitochondrial protein synthesis on the proliferation and phenotypic properties of a rat leukemia in different stages of in-vivo tumor development

It has been shown before that prolonged treatment with doxycycline (DC), an inhibitor of mitochondrial protein synthesis, leads to proliferation arrest of a leukemia in the rat and, moreover, to eradication of this tumor. It has also been demonstrated that the period of treatment required to achieve this is shorter when DC administration is started in later stages of tumor progression. Therefore, the leukemic cells may have properties with regard to DC sensitivity which change with time during tumor progression. In the present study this hypothesis was tested by studying the permeability for DC, the presence of cell-surface molecules, and the mitochondrial content of the leukemic cells in various stages of tumor development in control and in DC-treated rats. Changes in DC permeability or antigenic phenotype were not observed, but the content of mitochondria decreases during tumor progression. DC treatment leads to an additional reduction of the content of functional mitochondria which results in proliferation arrest. The higher mitochondrial content of the leukemic cells during the earlier stages of tumor development explains thus why a longer period of DC treatment is needed to achieve growth arrest when treatment is started in these stages.

The antitumour effect of doxycycline on a T-cell leukaemia in the rat

Previous studies showed that T-lymphoid cells are permeable to the tetracyclines, whereas B-lymphoid and erythroid cells are not. The tetracyclines impair mitochondrial protein synthesis if they have access to cells. Inhibition of mitochondrial protein synthesis during a number of cell cycles results, as a consequence, in proliferation arrest. The tetracyclines can therefore be considered as cytostatics. In the present study the effect of prolonged treatment with doxycycline on the growth of a T-cell type leukaemia of the rat was investigated. It is shown that doxycycline treatment inhibits not only tumour cell proliferation, but leads moreover to complete tumour eradication. The way by which the latter is achieved depends on the doxycycline concentration and, surprisingly, on the stage of tumour progression at which doxycycline administration is started. As, because of the permeability barrier, the proliferation of erythroid and B-lymphoid cells is not affected by the tetracyclines, the tetracyclines may provide a tool without serious side-effects in the therapy of T-type tumours.

Testicular invasion and relapse and meningeal involvement in a rat T-cell leukaemia

During the haematogenous dissemination of this acute rat T-cell (Roser) leukaemia, infiltration of both epididymal and testicular interstitial tissue has now been demonstrated, probably as an invariable occurrence. The gonadal duct system itself was not invaded. In contrast to an earlier histopathological study with this leukaemia, meningeal invasion has also been encountered during routine passage. Furthermore, subsequent to remissions induced by carmustine (BCNU), relapse could occur as long as 80 days after the 20 day end point in control animals. This was associated with extensive infiltration of the meninges as well as in the male gonadal interstitium, the proximal epididymis being particularly vulnerable. Two doses of carmustine at intervals of one week could eradicate the disease even during the phase of logarithmic growth of the leukaemic cells, this result depending upon the level of treatment and time of dosing post-inoculation with leukaemic cells. Females carrying the disease were shown to be more readily cured than males, probably related to entry of leukaemia cells into the gonadal interstitium. This T-cell leukaemia appears to be an excellent model for the study and prospective chemotherapy of testicular relapse in acute lymphoblastic leukaemia.

Blockade of the galactose-binding sites of ricin by its linkage to antibody. Specific cytotoxic effects of the conjugates

A method is described for preparing specific cytotoxic agents by linking intact ricin to antibodies in a manner that produces obstruction of the galactose-binding sites on the B chain of the toxin and so diminishes the capacity of the conjugate to bind non-specifically to cells. The conjugates were synthesised by reacting iodoacetylated ricin with thiolated immunoglobulin and the components of conjugate with reduced galactose-binding capacity were separated by affinity chromatography on Sepharose (a beta-galactosyl matrix) and asialofetuin-Sepharose. Fluorescence-activated cell sorter (FACS) analyses revealed that the fraction of a monoclonal anti-Thy1.1-ricin conjugate that passed through a Sepharose column had markedly diminished capacity to bind non-specifically to Thy1.2-expressing CBA thymocytes and EL4 lymphoma cells. The fraction of conjugate that passed through an asialofetuin-Sepharose column displayed no detectable non-specific binding. Both fractions of conjugate were potent cytotoxic agents for Thy1.1-expressing AKR-A lymphoma cells in tissue culture. They reduced the [3H]leucine incorporation of the cells by 50% at a concentration of 2-5 pM. Comparable inhibition of EL4 cells was only achieved with 3000-7500-fold greater concentrations of conjugate. By contrast, the fraction of anti-Thy1.1-ricin that retained Sepharose-binding capacity showed marked non-specific binding and toxicity to EL4 cells. A conjugate with diminished galactose-binding capacity was also prepared from the W3/25 monoclonal antibody which recognises an antigen upon helper T-lymphocytes in the rat. It elicited powerful and specific toxic effects upon W3/25 antigen-expressing rat T-leukaemia cells. This finding is of particular importance because isolated ricin A-chain disulphide-linked to W3/25 antibody is not cytotoxic. The property of the B-chain in intact ricin conjugates that facilitates delivery of the A-chain to the cytosol thus appears to be independent of galactose recognition. It is concluded that the 'blocked' ricin conjugates combine the advantages of high potency, which is often lacking in antibody-A-chain conjugates, with high specificity, which previously was lacking in intact ricin conjugates.

A quantitative study of the membrane antigens on leukaemic and normal T cells in the PVG rat

Membrane antigens on PVG leukaemic cells have been compared with those on normal PVG peripheral T cells by studying the capacity of the cells to bind antibodies against these cell types. The antibody bound has been quantitated by secondary binding of a radiolabelled anti-immunoglobulin. It was shown that the leukaemic cell lacks the peripheral thymus-derived lymphocyte antigens which the normal PVG peripheral T lymphocyte carries. In this respect, in its reduced distribution of the W3/13 antigen compared with T cells, as revealed by binding of the hybridoma monoclonal antibody W3/13, and in its abundance of Thy 1.1 antigen, the leukaemic cell resembles the immature thymocyte, rather than the mature T lymphocyte. This antigenic profile represents an illustration of de-differentiation of tumour cells at the molecular level. Two antigens shared by PVG leukaemic cells, normal PVG peripheral T cells and thymocytes were indicated by absorption studies with antisera against the leukaemic cell. In an immunotherapy model, treatment with AUG cells sensitized against the PVG leukaemia cell resulted in elimination of leukaemic cells from PVG rats. The peripheral T cell antigens clearly cannot be antigen(s) recognized by the AUG rat on the leukaemic cell, since the present studies indicate the absence of peripheral T cell antigens on this cell.

Effects of varying the interval between courses of methotrexate on its myelotoxic and anti-leukaemic activities

The toxicity produced by two courses of methotrexate separated by different intervals has been studied in matched groups of rats. The maximum degree of neutropenia reached when courses were separated by 8 days or more was no greater than that seen after a single course of methotrexate. However, when courses of neutropenia following the second course of methotrexate was directly related to the level of depression of bone marrow cell numbers at the time of the second course. Conversely the anti-leukaemic effects of 2 courses of methotrexate, in terms of time of onset of leukaemia and time of death in rats transplanted with a syngeneic T-cell leukaemia, are shown to be similar when courses of methotrexate are separated by between 2 and 12 days. Thus in this system, chemotherapeutic schedules using methotrexate may be designed on the basis of minimal host toxicity without prejudicing anti-leukaemic effects. These results are discussed in relation to toxicity and anti-leukaemic effects observed during UKALL trials of treatment in acute lymphoblastic leukaemia.

T cell leukaemia in the rat: stable marker chromosome in leukaemic cells

The karyotype of a transplantable T cell leukaemia which arose originally in a PVG strain rat undergoing chronic internal beta irradiation of the spleen was examined and a marker chromosome was found. Although the leukaemia progressively became more acute during 4 years of continuous passage the marker did not change. Cytogenetic analysis of normal rats given an acute exposure to external irradiation revealed, among other abnormalities, a marked of the same morphology indicating that the original marker was probably radiation induced. This is the first description of a stable marker chromosome in an experimental animal leukaemia.