Cyclophosphamide-facilitated adoptive immunotherapy of an established tumor depends on elimination of tumor-induced suppressor T cells.

On the basis of preceding studies showing that tumor-induced, T cell-mediated immunosuppression serves as an obstacle to adoptive immunotherapy of the Meth A fibrosarcoma, it was predicted that cyclophosphamide treatment of tumor bearers would remove this obstacle and allow passively transferred immune T cells to cause tumor regression. It was found that infusion of immune spleen cells alone had no effect on tumor growth, and cyclophosphamide alone caused a temporary halt in tumor progression. In contrast, combination therapy consisting of intravenous injection of 100 mg/kg of cyclophosphamide followed 1 h later by intravenous infusion of tumor-immune spleen cells caused small, as well as large tumors, to completely and permanently regress. Tumor regression caused by combination therapy was completely inhibited by intravenous infusion of splenic T cells from donors with established tumors, but not by spleen cells from normal donors. These suppressor T cells were eliminated from the spleen by treating the tumor-bearing donors with 100 mg/kg of cyclophosphamide. Immune T cells, in contrast, were resistant to this dose of cyclophosphamide. These results show that failure of intravenously-infused, tumor-sensitized T cells to cause regression of the Meth A fibrosarcoma growing in its syngeneic or semi-syngeneic host is caused by the presence of a tumor-induced population of cyclophosphamide-sensitive suppressor T cells.

Cellular kinetics associated with the development of acquired cellular resistance.

Mice infected with either BCG or Listeria monocytogenes display the same type of cellular response pattern. In both infections there is an intense proliferation of splenic lymphoid cells, a coincident proliferation of differentiated macrophages, and a subsequent production of macrophages with increased metabolic potential. The temporal relations between these events and the onset of host resistance indicate that lymphoid cell proliferation and macrophage proliferation are essential events in the response which leads to cellular resistance against facultative intracellular parasites.

The mitotic potential of fixed phagocytes in the liver as revealed during the development of cellular immunity.

Fixed macrophages in the liver of the mouse undergo mitosis in large numbers during an infection with Listeria monocytogenes. The macrophage mitotic response always precedes the expression of efficient host immunity to infection. It is suggested that macrophage proliferation is an important event in the development of host immunity to L. monocytogenes.

Suppression of cell-mediated immunity to infection by an antimitotic drug. Further evidence that migrant macrophages express immunity.

The development of acquired cell-mediated immunity to infection with Listeria monocytogenes can be blocked by a 15 hr pulse of the antimitotic drug, vinblastine (Vb). The drug has no effect on the host-parasite relationship after 72 hr of infection when a high level of immunity is being expressed, i.e., when infective foci are populated by activated macrophages. Infective foci in mice treated early during infection with Vb do not acquire migrant macrophages, but they become acellular after 48 hr of infection. The results indicate that Vb destroys the dividing precursors of migrant macrophages. The possibility that Vb prevents the activation of these cells by destroying dividing lymphoid cells engaged in the specific immunological phase of the host response is also considered.

The relative importance of blood monocytes and fixed macrophages to the expression of cell-mediated immunity to infection.

Infection in mice with Listeria monocytogenes results in a substantial accumulation of migrant macrophages in the liver. The immigrant cells populate both the infective foci and intervening sinusoids. They have the labeling characteristics of blood monocytes, and their appearance in infective foci in the liver corresponds to the expression of a high level of antimicrobial immunity in this organ. The infected liver acquires additional new macrophages by Kupffer-cell division. The proliferation of these cells, however, is not essential for the expression of immunity in the liver. The results indicate that the macrophages which express immunity to a primary infection with L. monocytogenes are those derived from circulating monocytes. Most of these cells are quickly lost once the parasite is eliminated from the tissues.

The action of cortisone acetate on cell-mediated immunity to infection. Suppression of host cell proliferation and alteration of cellular composition of infective foci.

Pulse labeling with tritiated thymidine shows that the response in the mouse to infection with L. monocytogenes includes a large increase in the division of lymphoid cells in the spleen, an increase in the division of macrophages in the liver, and an accumulation of monocyte-derived macrophages at infective foci in the tissues. A single 2.5 mg dose of cortisone acetate given at the beginning of infection greatly delays and suppresses these three components of the host response. The unrestricted bacterial multiplication which follows cortisone treatment is ultimately because of a failure of monocyte-derived macrophages to accumulate at infective foci where they normally express immunity. The accumulation of polymorphs at these sites, in contrast, is enhanced. It is argued that cortisone acetate prevents the accumulation of monocytes at infective foci indirectly by suppressing the production in the spleen of immunologically-committed lymphocytes which are needed to mediate the cellular events at infective foci.

Cellular mediators of anti-Listeria immunity as an enlarged population of short lived, replicating T cells. Kinetics of their production.

An intravenous immunizing infection with the facultative, intracellular parasite, Listeria monocytogenes results in the production in the spleen of a population of immunologically-committed lymphocytes which can adoptively immunize normal recipients against a lethal challenge infection. These cellular mediators of immunity are first produced in the spleen between days 2 and 4 of infection and reach peak production on day 6. Their production then progressively decreases until about day 20 when their presence can no longer be detected. Increased production of cellular mediators is coincident with major increases in cell division, cellularity, and spleen weight. Decreased production of cellular mediators, on the other hand, is associated with decreases in cell division, cellularity, and spleen weight. Again, the level of delayed sensitivity to Listeria antigens expressed by the host at any one time is proportional to the number of cellular mediators in the spleen. Increased production of cellular mediators is also associated with major increases in the total numbers of replicating T cells and B cells in the spleen. That the cellular mediators of immunity are part of the replicating T cell population, rather than the B cell population, is evidenced by their susceptibility to anti-theta serum and by their resistance to anti-Ig serum. Furthermore, they can be completely eliminated from the spleen by a brief pulse of the antimitotic drug, vinblastine. This study allows the conclusion that the cellular mediators of anti-Listeria immunity belong to an expanded population of rapidly dividing, short-lived T cells. It is suggested that they have the same properties as the T cell effectors of allograft immunity.

Radiation-induced, immunologically mediated regression of an established tumor as an example of successful therapeutic immunomanipulation. Preferential elimination of suppressor T cells allows sustained production of effector T cells.

The results of this study confirm results published by others by showing that sublethal whole-body irradiation of mice bearing immunogenic tumors can result in complete tumor regression. The results show, in addition, that irradiation-induced tumor regression can be prevented by infusion, after irradiation, of Ly-1+,2-,L3T4+ suppressor T cells from the spleens of donors bearing an established tumor, but not by infusion of normal spleen cells. This evidence, plus the demonstration that irradiation fails to cause regression of tumors growing in immunocompetent mice, is consistent with the hypothesis that irradiation-induced regression is immunologically mediated, and that it depends on the ability of irradiation to preferentially eliminate suppressor T cells. By using passive transfer assays to measure the production of effector T cells and suppressor T cells against time of tumor growth, it was shown that irradiation of tumor-bearing mice on day 5 of tumor growth resulted in a failure to generate suppressor T cells on the one hand, and in a sustained production, effector T cells on the other. In other words, irradiation prevented the concomitant antitumor immune response from being downregulated by suppressor T cells. However, giving radiation on day 1 of tumor growth, in contrast to giving it 3-6 d later, caused immunodepression and enhancement of tumor growth. This is in keeping with published evidence showing that, whereas resting effector T cells are highly radiosensitive, antigen-activated effector T cells are relatively radioresistant. It is suggested that the radioresistance of activated effector T cells, coupled with the radiosensitivity of activated suppressor T cells, is the reason for the selectivity of ionizing radiation for suppressor T cells and why a tumor needs to be palpable to undergo regression in response to radiation therapy.

Evidence inconsistent with a role for the Bcg gene (Nramp1) in resistance of mice to infection with virulent Mycobacterium tuberculosis.

The superior resistance of some strains of mice over others to infection with certain intracellular pathogens, including the vaccine strain of Mycobacterium bovis, bacillus Calmette Guerin (BCG), is determined by a gene associated with a small segment of chromosome 1 designated by Ity/Lsh/Bcg locus, referred to here as the Bcg locus. DBA/2 mice containing the dominant resistant allele of the Bcg gene (Bcgr), major histocompatibility complex-compatible BALB/c mice containing the recessive susceptible allele (Bcgs), and congenic C.D2-N20 Bcgr, which are genetically the same as BALB/c mice except for possessing a small piece of DBA/2 chromosome 1 containing the Bcg locus, were used to determine whether the Bcg gene determines resistance to infection with the virulent H37Rv strain of Mycobacterium tuberculosis (Mtb). According to the survival times of Bcgr and Bcgs mice infected via either the intravenous or respiratory route, Bcgr mice proved much less, rather than more, resistant to Mtb infection than Bcgs mice. Shorter survival times of Bcgr mice were associated with an inferior capacity to control Mtb growth in their lungs and to retard the development of Mtb-induced pathology in this organ. Resistance to Mtb infection was a dominant trait in the F1 progeny of Bcgr and Bcgs mice. The results show that resistance to Mtb is not determined by the resistance allele of the Bcg gene nor by the recently isolated candidate Bcg gene Nramp1, located in the Bcg locus.

T cell suppressors of antitumor immunity. The production of Ly-1-,2+ suppressors of delayed sensitivity precedes the production of suppressors of protective immunity.

The results of this study show that during growth of the immunogenic Meth A fibrosarcoma, two different types of suppressor T lymphocytes are generated in sequence. One type is generated during early tumor growth, reaches peak number around day 6, and is progressively lost thereafter. It is defined by its ability, upon passive transfer, to suppress the expression of a DTH reaction to tumor antigens in tumor-immunized recipients. It bears the Ly1-,2+ membrane phenotype and is sensitive to relatively low doses of cyclophosphamide. In contrast, the second type of suppressor cell is not detected until after day 9 of tumor growth, and is defined by its ability to inhibit, upon passive transfer, the expression of adoptive immunity against an established tumor in T cell-deficient recipients. According to previous studies it bears the Ly1+,2-, L3T4a+ membrane phenotype and is less sensitive to cyclophosphamide than the T cell suppressor of DTH. It is argued that this second type of suppressor T cell seems likely to be responsible for the escape of immunogenic tumors from antitumor immunity, because it can suppress the expression of a powerful mechanism of antitumor immunity in recipient mice, and is generated progressively as the tumor-bearing host loses concomitant immunity. In contrast, although the Ly-1-,2+ T cell suppressors of DTH can efficiently suppress a DTH reaction to an implant of living tumor cells, they fail to suppress the expression of immunity to the same implant.

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.

Generation and decay of the immune response to a progressive fibrosarcoma. I. Ly-1+2- suppressor T cells down-regulate the generation of Ly-1-2+ effector T cells.

It was shown that the progressive growth of the immunogenic meth A fibrosarcoma in its semisyngeneic host results in the generation of concomitant immunity to the growth of a tumor implant. The generation of immunity occurred between days 6 and 9 of tumor growth and was associated with the generation of sensitized T cells that were capable, on passive transfer, of causing regression of a 3-d tumor in gamma-irradiated recipients. After day 9 of tumor growth, concomitant immunity and the T cells able to passively transfer it were progressively lost, and this was associated with the generation of splenic suppressor T cells able to suppress the expression of adoptive immunity against an established tumor in T cell-deficient ( TXB ) recipients. The T cells that passively transferred concomitant immunity were shown to be of the Ly-1-2+ phenotype, in contrast to the T cells that transferred suppression, which were shown with the same reagents to be Ly-1+2-. The results are consistent with the hypothesis that the progressive growth of an immunogenic tumor results in the generation of Ly-1-2+-sensitized effector T cells that fail to reach a number sufficient to destroy the tumor because their generation is down-regulated by tumor-induced Ly-1+2- suppressor T cells.

Generation and decay of the immune response to a progressive fibrosarcoma. II. Failure to demonstrate postexcision immunity after the onset of T cell-mediated suppression of immunity.

This study shows that surgical removal of the meth A fibrosarcoma from its semisyngeneic host fails to result in postexcision immunity to growth of a tumor implant unless the host already has acquired a mechanism of concomitant immunity to growth of an implant. Therefore, tumor excision does not cause immunity to be generated but preserves a mechanism of concomitant immunity that already exists and which otherwise would eventually undergo down-regulation under the influence of suppressor T cells. Removal of the tumor after it has grown large enough to cause the T cell-mediated suppression of concomitant immunity does not result in the reemergence of immunity. Instead, the host remains unable to generate concomitant immunity to a second tumor for a long period of time and retains, for at least 31 d, suppressor T cells able to passively transfer suppression to appropriate recipients. Like the suppressor T cells responsible for active suppression of concomitant immunity, the suppressor T cells responsible for "memory" suppression are of the Ly-1+2- phenotype. The results indicate that progressive tumor growth results in a state of immunological tolerance of tumor-specific, transplantation antigens that can persist in the apparent absence of tumor antigens.

Mycobacterial virulence. Virulent strains of Mycobacteria tuberculosis have faster in vivo doubling times and are better equipped to resist growth-inhibiting functions of macrophages in the presence and absence of specific immunity.

The kinetics of growth of two virulent strains of mycobacteria (M. tuberculosis Erdman and M. tuberculosis H37Rv) and two attenuated strains (M. tuberculosis H37Ra and M. bovis Bacillus Calmette-Guerin [BCG]) were studied in the lungs, livers, spleens, and kidneys of severe combined immunodeficient (SCID) mice and of their coisogenic CB-17 immunocompetent counterparts. It was found, in keeping with the findings of earlier investigators (Pierce, C. H., R. J. Dubos, and W. B. Schaefer. 1953. J. Exp. Med. 97:189.), that in immunocompetent mice, virulent organisms grew progressively only in the lungs, whereas the growth of attenuated organisms was controlled in all organs. In SCID mice, in contrast, virulent mycobacteria grew rapidly and progressively in all organs, as did BCG, although at a slower rate. However, H37Ra failed to grow progressively in any organs of SCID mice, unless the mice were treated with hydrocortisone. In fact, hydrocortisone treatment enabled virulent, as well as attenuated, organisms to grow strikingly more rapidly in all organs of SCID mice and in all organs of CB-17 mice. A histological study showed that in SCID mice, multiplication of mycobacteria in the liver occurs in the cytoplasm of macrophages in granulomas and presumably in macrophages in other organs. It is suggested, therefore, that the macrophages of SCID mice possess a glucocorticoid-sensitive mycobacterial mechanism that prevents virulent and avirulent mycobacteria from expressing their true minimal doubling times. In the absence of this mechanism in the lungs of hydrocortisone-treated SCID mice, the doubling times of Erdman, H37Rv, BCG, and H37Ra were 17.7, 17.4, 44.6, and 98.6 h, respectively. The possible importance of a rapid multiplication rate for mycobacterial virulence is discussed.

Neutrophil-mediated dissolution of infected host cells as a defense strategy against a facultative intracellular bacterium.

The rate of growth of Listeria monocytogenes in the livers of mice infected intravenously with a lethal or sublethal inoculum of this facultative intracellular bacterium is greatly increased if neutrophils and other host cells are prevented from accumulating at foci of infection during the first 24 h by treatment with a monoclonal antibody (5C6) specific for the type 3 complement receptor of myelomonocytic cells. A histological examination of the livers of control mice showed that the accumulation of neutrophils at infectious foci resulted in the focal destruction of infected hepatocytes. In contrast, failure of neutrophils to accumulate at these sites in 5C6-treated mice allowed Listeria to multiply extensively in hepatocytes without destroying them. The results indicate that neutrophils play an important role in early defense against listeriosis in the liver by destroying infected hepatocytes, thereby reducing the opportunity for Listeria to multiply in permissive cells. In this way, neutrophils serve to break the chain of cell-to-cell spread of infection.

The antitumor function of tumor necrosis factor (TNF) II. Analysis of the role of endogenous TNF in endotoxin-induced hemorrhagic necrosis and regression of an established sarcoma.

In agreement with the results of previous studies (1), it was shown that intravenous injection of endotoxin into mice bearing 9-d SA1 sarcoma resulted in a tumor hemorrhagic reaction that rapidly caused necrosis of most of the center of the tumor, and then the complete regression of the rim of living tumor tissue that survived the hemorrhagic reaction. The tumor hemorrhagic reaction was confined to the vascular bed of the tumor, and its rate and extent of development were measured in terms of the intratumor extravasation of 51Cr-labeled syngeneic red cells. The development of the hemorrhagic reaction was associated with the presence in the tumor over a 6-h period of endogenous TNF that was measured in terms of its capacity to kill L929B cells in vitro and identified by its susceptibility to neutralization with a monospecific, polyvalent anti-rTNF antibody. The same antibody was capable in vivo of inhibiting the endotoxin-induced tumor hemorrhagic reaction by only approximately 50%, even when present in the tumor in excess. However, it was capable when given in the same quantity of inhibiting the ability of endotoxin to cause complete tumor regression. The fact that TNF was generated in the tumor during the tumor hemorrhagic reaction, and that infusion of a sufficient quantity of anti-rTNF antibody severely interfered with hemorrhagic necrosis and prevented tumor regression represents convincing evidence that TNF is an essential participant in endotoxin-induced regression of an established SA1 sarcoma. Moreover, because tumor regression, as opposed to hemorrhagic necrosis, failed to occur if the tumor was growing in immunoincompetent mice, but did so if the mice were infused with tumor-sensitized T cells, it can be concluded that an adequate level of T cell-mediated immunity is also an essential requirement for endotoxin-induced tumor regression. The participation of other endotoxin-induced mediators in tumor regression cannot be ruled out.

The antitumor function of tumor necrosis factor (TNF), I. Therapeutic action of TNF against an established murine sarcoma is indirect, immunologically dependent, and limited by severe toxicity.

The ability of murine recombinant tumor necrosis factor (rTNF) and natural TNF in tumor-necrotizing serum (TNS) to cause regression of the SA1 sarcoma was investigated. We found that to cause regression of a 9-d SA1 sarcoma, near lethal quantities of rTNF and TNS had to be given to the host. However, even at these highly toxic doses, rTNF was not reliable at causing complete tumor regression. On the other hand, both types of TNF were reliable at causing a tumor hemorrhagic reaction that resulted in the destruction of greater than 75% of the tumor's center in 24 h. The TNF-induced hemorrhagic reaction involved the development of numerous petechial hemorrhages in the tumor's vascular bed, which apparently resulted from destruction of the tumor's blood vessels. It was possible to follow the development of the hemorrhagic reaction against time after giving rTNF or TNS by measuring the intratumor extravasation of 51Cr-labeled syngeneic red cells. According to this method, TNF-induced intratumor hemorrhaging was in progress within 1 h of giving TNF and continued for about a 6-h period. However, the hemorrhagic reaction was greatly reduced and complete regression of the rim of the living tumor tissue that survived hemorrhagic necrosis failed to occur, if SA1 sarcoma was growing in T cell-deficient (TXB) mice. This indicates that the TNF-induced hemorrhagic reaction is partly dependent, and the tumor regression that follows is completely dependent on host immunocompetence. This suggests in turn, that rTNF does not directly destroy SA1 tumor cells in vivo, even though it was shown that it can destroy SA1 tumor cells in vitro. This interpretation is supported by the additional findings that rTNF was no more therapeutic against a 3-d (3-mm) SA1 than against a 9-d (8-mm) SA1, and was no more therapeutic when injected directly into the tumor than when injected intravenously. Lastly it was possible to completely inhibit the ability of rTNF and TNS to cause tumor hemorrhagic necrosis and regression by infusing the host with a monospecific, polyvalent anti-rTNF antibody that neutralized the cytotoxic action of rTNF in vitro.

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.

Exacerbation of murine listeriosis by a monoclonal antibody specific for the type 3 complement receptor of myelomonocytic cells. Absence of monocytes at infective foci allows Listeria to multiply in nonphagocytic cells.

Treatment of mice with a rat mAb (5C6) specific for an epitope of the type 3 complement receptor of myelomonocytic cells severely interfered with the ability of the mice to resist infection with Listeria monocytogenes. Consequently, a sublethal infection was rapidly converted to a lethal one that resulted in death in 5 d. However, infection was only exacerbated if 5C6 was given earlier in infection, before mononuclear phagocytes populated sites of Listeria implantation in the liver and spleen. If given after day 3 of infection, 5C6 caused only a temporary increase in bacterial multiplication. The infection-enhancing effect of 5C6 was associated with failure of mice to focus mononuclear phagocytes at sites of bacterial multiplication of Listeria in liver hepatocytes and extracellulary in the spleen. This resulted in unrestricted multiplication of Listeria in hepatocytes and extracellularly in the spleen. The results are in keeping with the ability of 5C6 to inhibit the accumulation of myelomonocytic cells in peritoneal inflammatory exudates, as revealed by a previous study.

Evidence for an alpha/beta T cell-independent mechanism of resistance to mycobacteria. Bacillus-Calmette-Guerin causes progressive infection in severe combined immunodeficient mice, but not in nude mice or in mice depleted of CD4+ and CD8+ T cells.

Depleting thymectomized mice of CD4+ T cells, or CD4+ plus CD8+ T cells, rendered them incapable of resolving Bacillus-Calmette-Guerin (BCG) infection in their lives, spleens, kidneys, and lungs. However, it did not render them incapable of stabilizing infection in the latter three organs after an initial period of BCG growth. Athymic nude mice showed a similar capacity to control BCG growth in these organs after a certain stage of infection. In contrast, congenitally severe combined immunodeficient (SCID) mice appeared to offer no resistance to BCG infection, in that the organism grew progressively in all organs of these mice and was lethal for them beginning on day 55 of infection. The results suggest that, although CD4+ T cells are important for resolving BCG infection, an alpha/beta T cell-independent mechanism of resistance can be acquired at 2-3 wk of infection that is capable of inhibiting further BCG growth in all organs except the lungs. Because this mechanism is absent from SCID mice, it is likely that it depends on the functions of gamma/delta T cells, B cells, or both types of cells. In keeping with this possibility is the additional finding that SCID mice engrafted with lymph node cells depleted of CD4+ or CD8+ T cells were capable of expressing an appreciable level of resistance against BCG infection.