Moloney leukemia virus-induced cell surface antigen mimicry by monoclonal antibodies.

We have investigated antigen-independent modulation of immune responses by monoclonal antibodies directed against both viral and nonviral antigens. BALB/c mice were immunized with monoclonal IgM (i.e. Ab1) specific for either Moloney murine leukemia virus-induced cell surface antigen (MCSA) or the hapten 2,4-dinitrophenyl (DNP). Injection with either Ab1 activated a functional idiotypic (Id) network as evidenced by production of both anti-Id (Ab2) antibodies and anti-anti-Id (Ab3) antibodies. A subset of induced Ab3 (designated Ab1'), exhibited specificity for antigen (virus or DNP). In mice immunized with anti-Id antibodies (Ab2), production of Ab3 and Ab1' was also observed. In the MCSA system, antibody-induced Ab1' responses were effective in protecting mice from tumor development upon subsequent challenge with live virus. Furthermore, antigen-independent modulation of immunity to both viral and nonviral antigens was found to be thymus-dependent. Similar findings in other viral systems suggest that antibody-induced activation of Id networks may prove a viable alternative vaccine strategy that can elicit antigen-specific responses, and in some cases protection, in the apparent absence of exposure to antigen.

Synergistic induction of thermotolerance in murine natural killer cells by interferon alpha and mild heat shock.

Splenic lymphocytes from C3H/HeN mice were primed in vivo or in vitro with the interferon inducer poly inosine:cytosine (Poly IC) or in vitro with interferon alpha (IFN alpha) and evaluated for their natural killer (NK) activity after exposure to hyperthermia for defined periods. Lytic activity against cells of the NK-susceptible Moloney lymphoma cell line YAC by Poly I:C- or IFN alpha-primed spleen cells exhibited thermotolerance to 41, 42 and 43 degrees C exposure compared to unprimed cells. Spleen cells were also incubated for 1 h at 40 or 37 degrees C prior to exposure to 42 degrees C. Incubation at 40 degrees C produced a modest increase in thermal resistance to 42 degrees C by otherwise unprimed spleen cells. Spleen cells that had been primed by Poly I:C or IFN alpha followed by 1 h at 40 degrees C were rendered even more resistant to hyperthermia at 42 degrees C. These data suggest that two host responses to viral infection, fever and production of IFN alpha, may endow cells involved in the inflammatory response (in this case NK cells) with resistance to more severe stress. Further, IFN alpha and fever may synergize in this protective mechanism.

A mechanism of retinoid potentiation of murine T-cell responses: early upregulation of interleukin-2 receptors.

The capacity of retinoids to amplify the proliferative response of BALB/c lymphocytes to concanavalin A (Con A)2 in the presence of exogenous interleukin-2 (IL-2) and the induction of IL-2 receptors (IL-2R) on L3T4+ and Lyt-2+ T-cells was evaluated. Preincubation with Con A for 8 h in the presence of retinoids resulted in a greater than two-fold increase in spleen cell proliferative response to Con A plus rIL-2 over the following 72 h relative to the response of cells preincubated with Con A alone. Peak potentiation of IL-2 responses occurred over a pharmacologic range of retinoic acid (RA) concentration (10(-10)-10(-8) M) in the presence of 20 U/ml rIL-2. This potentiation of the response to IL-2 was likewise observed after 8 h prestimulation with Con A with splenic T-cells enriched by passage over nylon wool. Preincubation of the spleen cells with Con A plus RA without the subsequent addition of IL-2 resulted in a proliferative response that was potentiated nearly to the level of the response produced by subsequent addition of IL-2 to Con A-activated cells. Preincubation of the cells with Con A in the presence of RA produced a true synergy with IL-2; the resulting increase in response was greater than the sum of the increases produced by RA or IL-2 alone.(ABSTRACT TRUNCATED AT 250 WORDS)

Detection of two early gene products of Epstein-Barr virus by fluorescence flow cytometry.

The intracellular expression of two early gene products of Epstein-Barr virus (EBV) was evaluated by fluorescence flow cytometry. Two Burkitt's lymphoma cell lines were superinfected by EBV from the P3HR-1 cell line. Twelve to 96 hr after superinfection the cells were fixed with paraformaldehyde and made permeable by saponin treatment. Monoclonal antibodies to the diffuse (D) and restricted (R) components of the early antigen (EA) complex were applied to the cells followed by fluorochrome-conjugated goat antibodies specific for the heavy chain isotypes of the monoclonal antibodies. Fluorescence flow cytometry revealed a clear separation in fluorescence intensity between cells containing EA-R or EA-D and negative cells. A major proportion of EA-positive cells displayed both antigens. In addition, a significant fraction expressed either EA-R or EA-D but not both. Expression of EA occurred more rapidly and peaked earlier in Daudi cells than in Raji cells. This apparent difference in EA expression between the two cell lines, however, was much less pronounced when one considered the proportion of cells expressing EA-R only, EA-D only, or both EA-R and EA-D, as a percentage of the total EA expression. Parallel fluorescence microscopy experiments revealed that the variation of the ratio of EA-R to EA-D expression with time correlated well between the two methods.

Potentiation of IL-2-induced t-cell proliferation by retinoids.

We evaluated the capacity of retinoids to potentiate proliferative responses of murine T-cells to recombinant human interleukin 2 (rIL-2). Concanavalin A (Con A) prestimulated spleen cells responded in a dose-dependent manner to added rIL-2. All-trans-retinoic acid (RA) at 10(-8) M potentiated the proliferative response by fivefold at saturating levels of IL-2. In similar experiments, two closely related retinamides, all-trans-(phenyl)retinamide (PR) and N-(4-hydroxyphenyl)retinamide (4-HPR), also potentiated murine splenocyte rIL-2 responses. Potentiation of IL-2-induced proliferation was dose-responsive to the concentration of added retinoid with peak potentiation occurring at 10(-10) - 10(-8) M in the presence of 10 U/ml rIL-2. Significant potentiation was observed at retinoid concentrations as low as 10(-14) M. Fluorescence flow cytometry of the responding cells revealed that among L3T4+, Lyt-2+ or total T-cells, at 72 h following Con A stimulation, essentially all of the cells expressed IL-2 receptors (IL-2R). This apparently represents near maximum IL-2R expression and treatment of the cells with retinoids did not increase IL-2R expression at that time point. The potentiation of IL-2 responses by retinoids was also observed with IL-2-dependent HT-2 cells, 98% of which were IL-2R positive. HT-2 proliferative responses to rIL-2 were potentiated as much as fourfold by 10(-10) M RA. HT-2 proliferative responses to rIL-2 were potentiated by all three retinoids dose dependently. Significant potentiation was observed with as little as 10(-14) M retinoid. Retinoids in the absence of IL-2 induced no proliferative responses. These data suggest that retinoids can augment the capacity of IL-2 to induce T-cell proliferation using Con A-activated murine splenic T-cell blasts and a long-term-cultured T-cell line.

Differential effects of human recombinant interferons on the expression of two early gene products of Epstein-Barr virus.

Two human Burkitt's lymphoma (BL) cell lines, Raji and Daudi, have been previously characterized as resistant and sensitive, respectively, to the anti-Epstein-Barr virus (EBV) effects of human leukocyte interferon. These cells are equally susceptible to P3HR-1 EBV superinfection as determined by EBV early antigen (EA) expression. The cell lines were pretreated with human recombinant interferons alpha 2, beta, or gamma and subsequently superinfected with P3HR-1 EBV. Their expression of two distinct EBV early gene products was evaluated by fluorescence microscopy. Monoclonal antibodies to the diffuse (EA-D) and restricted (EA-R) components of the EA complex were used to determine the number of cells expressing each of these antigens in the treated cell lines. As previously described with human leukocyte interferon, EA-D expression in Raji cells was relatively resistant to interferon-alpha 2 pretreatment. Also, EA-D expression in Daudi cells was relatively sensitive. However, interferon alpha 2 pretreatment produced an opposite pattern with respect to the expression of EA-R in these two cell lines; Raji cells were sensitive and Daudi cells relatively resistant. Interferon beta had the most uniformly effective anti-EBV activity on both cell lines; less than 15 U/ml produced 50% inhibition of both antigens in both cell lines. EA-D expression in both cell lines was sensitive to interferon-gamma pretreatment and EA-R was resistant. These data suggest that different gene products of EBV are independently regulated by interferons based on at least three factors: (1) the host cell, (2) the type of interferon and (3) the affected gene product.

Thymus-dependent network responses to a monoclonal cross-reactive antiidiotypic antibody.

BALB/c mice were inoculated i.p. with a cross-reactive anti-Idiotypic mAb (designated FD5-1) in the absence of Ag or adjuvants. Injection with unmodified FD5-1 resulted in the induction of serum antibodies reactive with both FD5-1 (Ab3) and the hapten DNP (Ab1'). Endpoint titers of the Ab3 response showed an increase in serum IgM, which was dose-responsive to both the number of injections and the amount of FD5-1 antibody injected. The serum IgM Ab3 response was found to be thymus dependent and idiotypically specific for FD5-1. Athymic mice injected with FD5-1 were unable to produce a serum IgM Ab3 response, whereas their euthymic littermates produced strong Ab3 responses. Serum Ab3 responses and Ab1' were detectable only in the IgM isotype; no specific IgG responses were observed. Indeed, IgG recognized by FD5-1 appeared to be suppressed by FD5-1. Injection of mice with FD5-1 modulated serum IgM responses to DNP, (4-hydroxy-3-nitrophenyl)acetyl (NP), 4-ethoxymethylene-2-phenyloxazol-5-one (OX), phosphorylcholine (PC), and alpha 1,3-dextran (DEX) in a thymus-dependent manner. FD5-1 injection induced IgM responses against DNP, (4-hydroxy-3-nitrophenyl)acetyl (NP), 4-ethoxymethylene-2-phenyloxazol-5-one, and DEX but decreased IgM binding to PC. No detectable Ab1' responses to any of the aforementioned molecules were found when the same sera were probed for IgG. The specificity of serum Ab1' from FD5-1-injected mice was evaluated by antigenic inhibition. Binding of serum Ab1' to DNP-BSA was inhibitable by DNP-lysine, whereas equivalent concentrations of lysine alone had no inhibitory effect. The antigenic specificity of IgM from normal serum binding to PC-BSA was demonstrated by inhibition with free PC, and the binding of Ab1' from FD5-1-injected mice to DEX-coated plates was shown to be inhibitable by DEX. We have described in vivo network perturbation in adult BALB/c mice injected with anti-Id antibody in the absence of Ag or adjuvants. Our findings show that injection of the cross-reactive anti-Id FD5-1 can induce thymus-dependent Ag-specific responses. In two systems where FD5-1 functions as an anti-anti-anti-Id antibody (PC and DEX), thymus-dependent responses were also observed. FD5-1 injection suppressed antibodies binding to PC, whereas DEX-specific responses were induced.

Differential effects of retinoids on pokeweed mitogen induced B-cell proliferation vs immunoglobulin synthesis.

The effects of three retinoids, all-trans-retinoic acid (RA), 13-cis-retinoic acid (cRA), and N-(4-hydroxyphenyl) retinamide (4-HPR) on mouse splenocyte responses to pokeweed mitogen (PWM) and Escherichia coli lipopolysaccharide (LPS) in vitro were evaluated. All three retinoids caused a dose dependent increase in the proliferative response to PWM. The retinoids hierarchy of efficacy based on potentiation of PWM-induced splenocyte proliferation was RA greater than cRA greater than 4-HPR. 13-cis-retinoic acid and 4-HPR also resulted in significant increases in Ig secretion in response to PWM. However, RA did not produce a significant increase in secretion compared with cells treated with PWM alone. The efficacy hierarchy of retinoids ability to potentiate Ig secretion was 4-HPR greater than cRA greater than RA. All three compounds did not affect Ig secretion from LPS-stimulated splenocytes and produced dose dependent decreases in proliferation. Both inhibition of LPS-induced proliferation and potentiation of PWM-induced proliferation were maximal when the retinoids were added during the first hour of culture. These results indicate that retinoids have a differential effect on Ig secretion and B-cell proliferation based on structural differences of the retinoids. Potentiation of proliferation and Ig secretion are both T-cell dependent and could be a result of increased lymphokine synthesis by the T-cells or increased responsiveness to the effects of the T-cell produced lymphokines.

Thymus-dependent antiidiotype and anti-antiidiotype responses to a dinitrophenyl-specific monoclonal antibody.

BALB/c mice were inoculated i.p. with graded doses of a DNP-specific, IgM mAb (designated 57.1). Injection with unmodified 57.1 in the absence of adjuvants resulted in the generation of an anti-Id response (Ab2) and an anti-anti-Id response (Ab3). The generation of serum anti-Id antibodies was found to be thymus dependent. Nude mice immunized with 57.1 were unable to produce a serum Ab2 response above nonimmunized controls whereas euthymic mice receiving identical doses of 57.1 produced strong Ab2 responses. To examine the specificity of serum anti-Id, sera from mice receiving 57.1 were screened against a panel of mAb representing at least five distinct VH gene families. Serum titers were significantly higher against 57.1 than against any of the other antibodies in the panel. Three of the antibodies in this panel bind FD5-1, a monoclonal anti-Id (Ab2) that also binds 57.1. However, sera from mice receiving 57.1 bound 57.1 only. Thus, the serum Ab2 response appears to be highly specific for idiotopes on 57.1. The predominant isotype of these anti-Id antibodies was IgG1. The number of isotypes detected increased in a dose dependent manner with all IgG subclasses having anti-Id specificity in sera from animals receiving the higher doses of 57.1. Further analysis of the serum demonstrated that approximately 8% of the Ab2 response was paratope-specific (inhibitable by the monovalent hapten DNP-lysine). The same sera were analyzed for the presence of Ab3 by binding to the monoclonal anti-Id antibody FD5-1. Lower serum titers of Ab3 were generated in comparison to serum titers of Ab2. Analysis of the binding specificity of the Ab3 response revealed that DNP-BSA was able to partially inhibit the binding of serum IgM and IgG Ab3 to FD5-1. A subset of the Ab3 response. Ab1' that is specific for DNP was observed in a direct binding assay where detectable amounts of DNP binding IgM, IgG1, and IgG3 isotypes were present. We have thus described a complete circuit (Ab1----Ab2----Ab3) of antibodies within the Id network by immunizing animals with an unmodified mAb in the absence of Ag or adjuvants.

Pharmacological and toxicological properties of arotinoids SMR-2 and SMR-6 in mice.

Studies were conducted to define primary pharmacological and toxicological properties of two arotinoids, SMR-2 and SMR-6, in male B6D2F1 mice. Mice were gavaged daily for up to 22 days with retinoids in corn oil (0.1, 0.2, or 0.4 mg/kg day SMR-2 or SMR-6 or 2.5, 10, or 30 mg/kg all-trans-retinoic acid as a reference control). Toxicological and biochemical endpoints were assayed after 8, 15, and 22 days. At toxic doses, i.e., those inducing weight loss, morphological changes were observed in skin, lymph nodes, spleen, bone marrow, liver, thymus, forestomach, adrenal, bone, and testes. Biochemical alterations included elevated serum alkaline phosphatase, corticosterone, and interleukins-1, -2, and -3. Additional immune alterations included increased responsiveness of spleen cells to both thymus-dependent and thymus-independent mitogens and increases in the total number of B cells in the spleen. At doses not inducing weight loss, target organ effects included the appearance of plasma cells and infiltration of polymorphonuclear cells in lymph nodes; myeloid cell hypercellularity in bone marrow; hematopoiesis in spleen; subacute inflammation in forestomach; and periportal cytoplasmic vacuolization in liver. At the low doses, SMR-2 resulted in decreased responsiveness of spleen cells to mitogens and SMR-6 caused increased responsiveness. SMR-6 also increased interleukin-1 and-2 production at low doses. Biochemical effects included reduced activities of liver aryl hydrocarbon hydroxylase (AHH) and soluble brain protein kinase C. Overall, the results suggest that leukopoiesis and reduced liver AHH and reduced soluble protein kinase C activities are the primary and initial pharmacological and toxicological effects of retinoids.

In vitro effect of acetaldehyde on cell-mediated cytotoxicity by murine spleen cells.

The effects of acetaldehyde in vitro on the lytic capacity of murine spleen cells have been evaluated in three systems: antibody-dependent cell-mediated cytotoxicity (ADCC), natural killer (NK) activity, and alloimmune cytotoxic T lymphocyte (CTL) activity. Acetaldehyde had a biphasic effect on ADCC. Concentrations less than 1 mM acetaldehyde potentiated ADCC. Concentrations greater than 1 mM produced a progressive decrease in lysis. The inhibitory effects were at the effector cell level and were partially irreversible. Preincubation experiments showed that inhibition of ADCC was both concentration and time-dependent. Preincubation of the spleen cells for short periods of time produced potentiated lysis by concentrations of acetaldehyde up to 10 mM. However, potentiation of lysis in preincubation and short term (4h) lytic assay experiments was more variable than longer term (18h) experiments in which the acetaldehyde was not removed by washing. NK activity and alloimmune CTL-mediated lysis were also inhibited by acetaldehyde. Concentrations of acetaldehyde up to 20 mM did not significantly decrease lymphocyte viability as determined by trypan blue exclusion. Acetaldehyde was lost from the reaction mixtures by first order kinetics with a rate constant of 0.5/hr. Thus, the final concentrations were 64-99.99% lower than the starting amounts.

Ethanol inhibition of cell-mediated lysis of antibody-sensitized target cells at a calcium-dependent step.

Ethanol inhibits antibody-dependent cell-mediated cytotoxicity in a dose-dependent manner. The inhibitory effect of ethanol was reversed by the addition of excess calcium or calcium ionophore A23187. Excess calcium at 4-8 mM concentrations was required to reverse 50% of the inhibition caused by ethanol. In seven of nine experiments, 16 mM excess calcium completely reversed the inhibition and produced greater lysis than the control. Excess calcium in the absence of ethanol induced a dose-dependent increase in lytic activity by the spleen cells. However, the reversal of inhibition by ethanol could not be attributed to a simple additive effect resulting from the increased cytolytic capacity of the lymphocytes in the presence of excess Ca2+. Ionophore A23187 at 1 microM also partially reversed the inhibitory effect caused by ethanol. Ionophore alone did not potentiate lytic activity. When target cells were not sensitized with antibody, excess calcium had no effect on the lysis of target cells in the presence of ethanol-treated or untreated lymphocytes. These data suggest that ethanol inhibits antibody-dependent cell-mediated cytotoxicity at a calcium-dependent step.

Induction of effective immunity to Moloney murine sarcoma virus using monoclonal anti-idiotypic antibody as immunogen.

We have isolated an anti-idiotypic mAb (RS1.1.3), which recognizes an idiotope present on several IgM mAb specific for Moloney murine leukemia virus (M-MuLV)-determined cell surface Ag. The binding of RS1.1.3 to idiotypic antibody could be inhibited by specific Ag. Intraperitoneal immunization of mice with purified RS1.1.3 antibody-induced effective immunity against Moloney murine sarcoma virus challenge. A single injection of RS1.1.3 7 days before virus challenge resulted in a 27% reduction in tumor load compared to non-immune control mice challenged with the same dose of virus, whereas multiple injections of RS1.1.3 before virus challenge resulted in a 75% reduction in tumor load. The protective effect of anti-idiotype immunization appeared to be T dependent, because immunization of athymic mice had no effect on their susceptibility to tumor virus challenge. Administration of the anti-idiotypic antibody after virus inoculation caused an increase in tumor load of nearly 50% compared to non-immune controls. BALB/c mice immunized with RS1.1.3 developed anti-anti-idiotypic antibodies, as well as M-MuLV Ag-specific antibodies. Analysis of sera from RS1.1.3-immune mice subsequently challenged with Moloney murine sarcoma virus indicated an inverse relationship between tumor load and M-MuLV-specific serum IgG titers induced by the RS1.1.3 immunization. These results indicate that anti-idiotypic mAb may be used as immunogen to induce Ag-specific antibody responses, and to cause effective immunity to a retro-virus-induced tumor.

Isotype distribution and specificity of the antibody response to primary Moloney murine sarcoma virus infection in BALB/c mice.

The development and isotype distribution of Moloney murine leukemia virus (M-MuLV)-specific serum antibodies following primary inoculation with Moloney murine sarcoma/leukemia virus (M-MuSV/M-MuLV) in adult BALB/c mice have been investigated using an enzyme-linked immunosorbent assay (ELISA). The primary antibody responses to M-MuSV/M-MuLV consisted of the IgM, IgG2a, IgG2b, and IgG3 isotypes; no M-MuLV-specific serum IgG1 or IgA antibodies were detected. The detectable antibody response was biphasic, with an early peak of virus-specific titers seen between 10 and 15 days after inoculation and a second peak seen in regressor sera. Pooled regressor sera contained IgM, IgG2a, and IgG2b antibodies which bound to M-MuLV-expressing lymphoma cells. Immunoelectron microscopy with regressor sera showed IgG bound both to infected cell surfaces and to mature viral particles, while IgM bound only to infected cell surfaces. These findings were supported by immunoprecipitation analyses which demonstrated binding of the M-MuLV-specific antibodies to both virion-associated and cell-associated antigens encoded by the gag and env genes.

Effects of retinoids on human thymus-dependent and thymus-independent mitogenesis.

The effects of all-trans-retinoic acid (RA), 13-cis-retinoic acid, and N-(4-hydroxyphenyl)retinamide on the mitogenic responses of various populations of human lymphocytes have been evaluated. Superoptimal concentrations of mitogens allowed the greatest RA-induced potentiation of lymphocyte proliferation. All three retinoids at concentrations as low as 5 x 10(-14)M significantly potentiated the proliferation of adenoidal and tonsillar lymphocytes stimulated by pokeweed mitogen (PWM). However, the responses of adenoidal and tonsillar lymphocytes to Staphylococcus aureus Cowan strain A were not potentiated by retinoids. Retinoids also caused significant potentiation of proliferation of PWM-stimulated peripheral blood lymphocytes (PBL). However, endpoint concentrations of retinoids required to significantly potentiate PBL proliferative responses to PWM were much higher than required for potentiation of adenoidal or tonsillar lymphocytes. PBL responses to concanavalin A (Con A) were significantly potentiated by retinoid concentrations as low as 10(-8) to 10(-10) M. Retinoid-potentiated responses were also observed wi Con A-stimulated thymocytes, but the endpoint concentrations required for significant potentiation were 10-fold higher than required to potentiate PBL responses to Con A. These data indicate that the sensitivity of lymphocytes to the retinoid-mediated potentiation of mitogenesis depends on the lymphoid compartment from which the cells are obtained. Tonsillar and adenoidal lymphocytes were the most responsive of the lymphocytes tested to the retinoid-induced potentiation of PWM responses. In addition, retinoids appear to selectively potentiate T cell-dependent proliferative activity.

In vitro effects of ethanol and acetaldehyde on lymphoid cells as determined by 31P NMR spectroscopy.

We have used NMR spectroscopy to test the direct in vitro effects of ethanol and its metabolite acetaldehyde on a human leukemic T cell line (Molt-4) and normal murine spleen cells. The metabolic status of phosphate metabolites (phosphomonoesters including sugar phosphate, inorganic phosphate and ATP's) in cell suspension was monitored by 31P NMR spectroscopy. Spectral changes were observed in the intensity of inorganic phosphate and ATP. Addition of high concentrations of ethanol (400-1600 mM) to Molt-4 cells resulted in very little spectral change. However, the addition of 40 microM acetaldehyde resulted in substantially increased inorganic phosphate (Pi) signals, and decreased phosphomonoesters and ATP signals. Similar changes, but to a lesser degree, were observed with normal mouse spleen cells.

Inhibition of Moloney murine lymphoma and sarcoma growth in vivo by dietary retinoids.

The effects of dietary retinoids on the growth of Moloney lymphoma (LSTRA) and sarcoma (MSC) in BALB/c mice were evaluated. Transplantable syngeneic Moloney lymphoma and sarcoma tumors are immunogenic. Preimmunization with LSTRA cells provides protection against subsequent challenge and sarcomas spontaneously regress following injection of an appropriate inoculum of MSC cells. In normal mice fed varying concentrations of all-trans-retinoic acid (RA) and given injections of 10(3) LSTRA cells, RA caused a dose-dependent increase in the number of survivors; 50% of the mice fed RA at 50 mg/kg of diet were long-term survivors. All animals died that were fed a control diet and challenged with 10(3) LSTRA cells. Athymic (nu/nu) mice fed RA were not protected against lymphoma growth, whereas euthymic (nu/+) mice were; therefore, the antitumor effect of RA was thymus dependent. Primary immunization with irradiated LSTRA in the presence of RA caused a significant increase in cell-mediated cytotoxicity by spleen cells at 4 days after immunization. However, challenge of animals preimmunized with LSTRA in the presence of dietary RA revealed a dose-dependent inhibition of memory. A significant reduction in MSC growth was also observed in normal mice fed 13-cis-retinoic acid (cRA). A comparison of the primary antilymphoma effect of dietary RA, cRA, N-(all-trans-retinoyl)-DL-leucine (RL), and N-(4-hydroxyphenyl)retinamide (4-HPR) revealed an efficacy hierarchy of RL greater than RA greater than cRA greater than 4-HPR with RL producing 70% long-term survivors at 115 days after challenge with 10(3) LSTRA cells. These studies indicate that retinoids can inhibit the growth of transplantable, retroviral-induced, immunogenic tumors by thymus-dependent mechanisms and that a newly synthesized retinoylamino acid (RL) is more potent than RA at inhibiting Moloney lymphoma growth.