CD8+ CTLs are essential for protective immunity against Encephalitozoon cuniculi infection

Encephalitozoon cuniculi is a protozoan parasite that has been implicated recently as a cause of opportunistic infection in immunocompromised individuals. Protective immunity in the normal host is T cell-dependent. In the present study, the role of individual T cell subtypes in immunity against this parasite has been studied using gene knockout mice. Whereas CD4-/- animals resolved the infection, mice lacking CD8+ T cells or perforin gene succumbed to parasite challenge. The data obtained in these studies suggest that E. cuniculi infection induces a strong and early CD8+ T response that is important for host protection. The CD8+ T cell-mediated protection depends upon the CTL activity of this cell subset, as the host is rendered susceptible to infection in the absence of this function. This is the first report in which a strong dependence upon the cytolytic activity of host CD8+ T cells has been shown to be important in a parasite infection.     

Plasmacytoid DC from aged mice down-regulate CD8 T cell responses by inhibiting cDC maturation after Encephalitozoon cuniculi infection

Age associated impairment of immune function results in inefficient vaccination, tumor surveillance and increased severity of infections. Several alterations in adaptive immunity have been observed and recent studies report age related declines in innate immune responses to opportunistic pathogens including Encephalitozoon cuniculi. We previously demonstrated that conventional dendritic cells (cDC) from 9-month-old animals exhibit sub-optimal response to E. cuniculi infection, suggesting that age associated immune senescence begins earlier than expected. We focused this study on how age affects plasmacytoid DC (pDC) function. More specifically how aged pDC affect cDC function as we observed that the latter are the predominant activators of CD8 T cells during this infection. Our present study demonstrates that pDC from middle-aged mice (12 months) suppress young (8 week old) cDC driven CD8 T cell priming against E. cuniculi infection. The suppressive effect of pDC from older mice decreased maturation of young cDC via cell contact. Aged mouse pDC exhibited higher expression of PD-L1 and blockade of their interaction with cDC via this molecule restored cDC maturation and T cell priming. Furthermore, the PD-L1 dependent suppression of cDC T cell priming was restricted to effector function of antigen-specific CD8 T cells not their expansion. To the best of our knowledge, the data presented here is the first report highlighting a cell contact dependent, PD-L1 regulated, age associated defect in a DC subpopulation that results in a sub-optimal immune response against E. cuniculi infection. These results have broad implications for design of immunotherapeutic approaches to enhance immunity for aging populations.     

Heterosubtypic immunity to influenza A virus in mice lacking IgA, all Ig, NKT cells, or gamma delta T cells

The mechanisms of broad cross-protection to influenza viruses of different subtypes, termed heterosubtypic immunity, remain incompletely understood. We used knockout mouse strains to examine the potential for heterosubtypic immunity in mice lacking IgA, all Ig and B cells, NKT cells (CD1 knockout mice), or gamma(delta) T cells. Mice were immunized with live influenza A virus and compared with controls immunized with unrelated influenza B virus. IgA(-/-) mice survived full respiratory tract challenge with heterosubtypic virus that was lethal to controls. IgA(-/-) mice also cleared virus from the nasopharynx and lungs following heterosubtypic challenge limited to the upper respiratory tract, where IgA has been shown to play an important role. Ig(-/-) mice controlled the replication of heterosubtypic challenge virus in the lungs. Acute depletion of CD4+ or CD8+ T cell subsets abrogated this clearance of virus, thus indicating that both CD4+ and CD8+ T cells are required for protection in the absence of Ig. These results in Ig(-/-) mice indicate that CD4+ T cells can function by mechanisms other than providing help to B cells for the generation of Abs. Like wild-type mice, CD1(-/-) mice and gamma(delta) (-/-) mice survived lethal heterosubtypic challenge. Acute depletion of CD4+ and CD8+ cells abrogated heterosubtypic protection in gamma(delta) (-/-) mice, but not B6 controls, suggesting a contribution of gamma(delta) T cells. Our results demonstrate that the Ab and cellular subsets deficient in these knockout mice are not required for heterosubtypic protection, but each may play a role in a multifaceted response that as a whole is more effective than any of its parts.     

CD8+ T lymphocytes protect SCID mice against Encephalitozoon cuniculi infection

Microsporidia are obligate intracellular parasites that cause opportunistic infections in immunocompromised patients. The role of two main T cell subsets in anti-microsporidial immunity has been studied using an Encephalitozoon cuniculi-severe combined immunodeficient (SCID) mouse model. Whereas SCID mice reconstituted with CD4+ T lymphocyte-depleted naive BALB/c splenocytes resolved the infection, adoptive transfer of CD8+ T cell-depleted splenocytes failed to protect the animals against a lethal E. cuniculi infection. Splenocytes from E. cuniculi-immune mice specifically killed syngeneic infected macrophages in a short-term 51Cr-release assay. These results suggest the crucial role of cytotoxic T lymphocytes in the protection against E. cuniculi infection.     

Augmentation of the CD8+ T cell response by IFN-gamma in IL-12-deficient mice during Toxoplasma gondii infection

The importance of IFN-gamma in regulating the host CD8+ T cell response during microbial infection has not been delineated. Mice deficient for the p40 chain of the IL-12 heterodimer have impaired IFN-gamma production and are susceptible to infection with the intracellular parasite Toxoplasma gondii. The administration of exogenous IFN-gamma to parasite-infected p40-/- mice increases survival and up-regulates the depressed CD8+ T cell response following infection. CD8+ T cells isolated from cytokine-treated p40-/- mice exhibit an increase in both precursor CTL frequency and IFN-gamma production compared with untreated controls. The enhancement of the CD8+ T cell response is independent of CD4+ T cell help. These CD8+ T cells induce protective immunity against a lethal challenge when adoptively transferred into naive p40-/- and IFN-gamma-/- mice. These observations indicate that IFN-gamma can regulate the CD8+ T cell response during T. gondii infection.     

Selective regulation of CD8 effector T cell migration by the p110 gamma isoform of phosphatidylinositol 3-kinase

Chemokine-mediated T cell migration is essential to an optimal immune response. The p110gamma isoform of PI3K is activated by G protein-coupled receptors and regulates neutrophil and macrophage chemotaxis. We used p110gamma-deficient mice to examine the role of p110gamma in CD8 T cell migration and activation in response to viral challenge. Naive CD8 T cell migration in response to CCL21 in vitro and trafficking into secondary lymphoid organs in vivo was unaffected by the loss of p110gamma. Furthermore, loss of p110gamma did not affect CD8 T cell proliferation and effector cell differentiation in vitro in response to anti-CD3 stimulation or in vivo in response to vaccinia virus (VV) challenge. However, there was reduced migration of p110gamma knockout (p110gamma(-/-)) CD8 effector T cells into the peritoneum following i.p. challenge with VV. The role of p110gamma in CD8 effector T cell migration was intrinsic to T cells, as p110gamma(-/-) CD8 effector T cells exhibited impaired migration into the inflamed peritoneum following secondary transfer into wild-type recipients. In addition, p110gamma(-/-) CD8 effector T cells exhibited impaired migration in vitro in response to inflammatory chemoattractants. Although wild-type mice efficiently cleared VV at high viral doses, infection of p110gamma knockout mice resulted in visible illness and death less than a week after infection. Thus, p110gamma is dispensable for constitutive migration of naive CD8 T cells and subsequent activation and differentiation into effector CD8 T cells, but plays a central role in the migration of effector CD8 T cells into inflammatory sites.     

IL-15 prolongs the duration of CD8+ T cell-mediated immunity in mice infected with a vaccine strain of Toxoplasma gondii.

Immunization of mice with a vaccine (ts-4) strain of Toxoplasma gondii is known to induce complete protection against subsequent lethal infection. Ts-4-mediated protection has been reported to be primarily dependent on IFN-gamma-producing CD8+ T cells. However, duration of CD8+ T cell-mediated immunity in the ts-4-vaccinated animals is not known. In the present study, the kinetics of the CD8+ T cell response in mice immunized with the ts-4 strain of T. gondii was evaluated. Optimal CD8+ T cell immunity persisted at least 6 mo after vaccination, and mice at this time point continued to overcome lethal challenge with a more virulent strain. However, at 9 mo postimmunization, CD8+ T cell immunity was severely diminished and the mice succumbed to Toxoplasma challenge. Pretreatment of animals, vaccinated 9 mo earlier, with rIL-15 prevented the mortality induced by Toxoplasma challenge. The protective effect of IL-15 treatment was due to a rise in the frequency of Ag-specific CD8+ T cells. CD8+ T cells from IL-15-administered animals showed increased proliferation and IFN-gamma production in response to antigenic restimulation. These findings suggest that rIL-15 can reverse the decline in the long-term CD8+ T cell immune response in mice immunized with vaccine strain of T. gondii.     

Comparison of the significance of CD4+ and CD8+ T lymphocytes in the protection of mice against Encephalitozoon cuniculi infection

The role of T lymphocyte subpopulations in the protection against intraperitoneal (i.p.) and peroral Encephalitozoon cuniculi infections was compared in adoptive-transfer experiments using severe combined immunodeficient mice. Whereas CD8+ T cell-depleted, but not CD4+ T cell-depleted, BALB/c splenocytes failed to protect the mice against i.p. infection, only SCID mice reconstituted with both CD4+ T lymphocyte- and CD8+ T lymphocyte-depleted splenocytes succumbed to peroral infection. The results indicate that whereas CD8+ T cells are critical for the protection against an i.p. E. cuniculi infection, both CD4+ and CD8+ T lymphocyte subpopulations play a substantive protective role in a peroral infection, i.e., natural route of infection.     

Adaptive immunity and enhanced CD8+ T cell response to Listeria monocytogenes in the absence of perforin and IFN-gamma

Single Ag-specific CD8+ T cells from IFN-gamma-deficient (GKO) or perforin-deficient (PKO) mice provide substantial immunity against murine infection with Listeria monocytogenes. To address the potential for redundancy between perforin and IFN-gamma as CD8+ T cell effector mechanisms, we generated perforin/IFN-gamma (PKO/GKO) double-deficient mice. PKO/GKO-derived CD8+ T cells specific for the immunodominant listeriolysin O (LLO91-99) epitope provide immunity to LM infection similar to that provided by Ag-matched wild-type (WT) CD8+ T cells in the liver but reduced in the spleen. Strikingly, polyclonal CD8+ T cells from immunized PKO/GKO mice were approximately 100-fold more potent in reducing bacterial numbers than the same number of polyclonal CD8+ T cells from immunized WT mice. This result is probably quantitative, because the frequency of the CD8+ T cell response against the immunodominant LLO91-99 epitope is >4.5-fold higher in PKO/GKO mice than WT mice at 7 days after identical immunizations. Moreover, PKO/GKO mice can be immunized by a single infection with attenuated Listeria to resist >80,000-fold higher challenges with virulent organisms than naive PKO/GKO mice. These data demonstrate that neither perforin nor IFN-gamma is required for the development or expression of adaptive immunity to LM. In addition, the results suggest the potential for perforin and IFN-gamma to regulate the magnitude of the CD8+ T cell response to infection.     

Early appearing gamma/delta-bearing T cells during infection with Calmétte Guérin bacillus.

To search for a potential role of TCR gamma/delta T cells in host-defense against mycobacterial infection, we analyzed the kinetics, repertoire, specificity, and cytokine production of gamma/delta T cells in the peritoneal exudate cells (PEC), lymph node (LN) cells and spleen cells during an i.p. infection with a sublethal dose (5 x 10(5) of viable Bacillus Calmétte-Guérin (BCG) in mice. In the PEC on day 7 after infection, approximately 26% of the CD3+ cells were CD4-CD8-, most of which expressed TCR gamma/delta on their surface. However, the PEC on day 28 contained an increased number of alpha/beta T cells that were CD4+8- or CD4-8+ and the proportion of gamma/delta T cells in the PEC reciprocally decreased to 18% of the CD3+ cells. The kinetics of gamma/delta and alpha/beta T cells in the LN during BCG infection showed in much the same pattern as that seen in the PEC. When purified CD4-CD8- cells in the LN on day 7 after BCG infection were cultured with sonicated BCG lysate, PPD derived from Mycobacterium tuberculosis or recombinant 65 kDa heat shock protein derived from Mycobacterium bovis, the gamma/delta T cells on this stage significantly proliferated and secreted IL-2 in response to sonicated BCG lysate and PPD but not to 65 kDa heat shock protein. V gene segment usage analysis with PCR method revealed that purified protein derivative-reactive gamma/delta T cells preferentially used V gamma 1/2/V delta 6, whereas gamma/delta T cells polyclonally expanded in response to the BCG lysate. These results suggest that gamma/delta T cells specific for mycobacterial antigens preceding alpha/beta T cells in appearance during infection may serve as a first line of defense against mycobacterial infection.     

Complement-dependent enhancement of CD8+ T cell immunity to lymphocytic choriomeningitis virus infection in decay-accelerating factor-deficient mice

Decay-accelerating factor (DAF, CD55) is a GPI-anchored membrane protein that regulates complement activation on autologous cells. In addition to protecting host tissues from complement attack, DAF has been shown to inhibit CD4+ T cell immunity in the setting of model Ag immunization. However, whether DAF regulates natural T cell immune response during pathogenic infection is not known. We describe in this study a striking regulatory effect of DAF on the CD8+ T cell response to lymphocytic choriomeningitis virus (LCMV) infection. Compared with wild-type mice, DAF knockout (Daf-1(-/-)) mice had markedly increased expansion in the spleen of total and viral Ag-specific CD8+ T cells after acute or chronic LCMV infection. Splenocytes from LCMV-infected Daf-1(-/-) mice also displayed significantly higher killing activity than cells from wild-type mice toward viral Ag-loaded target cells, and Daf-1(-/-) mice cleared LCMV more efficiently. Importantly, deletion of the complement protein C3 or the receptor for the anaphylatoxin C5a (C5aR) from Daf-1(-/-) mice reversed the enhanced CD8+ T cell immunity phenotype. These results demonstrate that DAF is an important regulator of CD8+ T cell immunity in viral infection and that it fulfills this role by acting as a complement inhibitor to prevent virus-triggered complement activation and C5aR signaling. This mode of action of DAF contrasts with that of CD59 in viral infection and suggests that GPI-anchored membrane complement inhibitors can regulate T cell immunity to viral infection via either a complement-dependent or -independent mechanism.     

Gamma delta T cells assist alpha beta T cells in adoptive transfer of contact sensitivity.

Cutaneous immune responses to contact sensitizers such as picryl chloride or oxazolone, are classical manifestations of T cell-mediated immunity in vivo. In fact, the first documentation of T cell-mediated immunity was the ability to adoptively transfer contact sensitivity (CS) responses. Although it is now clear that Ag/MHC-restricted alpha beta TCR positive effector T cells are responsible for 24 to 48 h CS responses, other subsets of Thy-1+ cells in mice also participate in the elicitation of CS. Thus, Thy-1+, CD5+, CD3-, B220+, hapten-specific, non-MHC-restricted early-acting cells are required to initiate CS responses by leading to local serotonin release, which allows for extravascular recruitment of the late-acting, alpha beta TCR+, CS effector T cells. This study describes another T cell population that is needed for the adoptive transfer of CS by alpha beta T cells. In vitro treatment of a mixture of CS effector cells with hamster mAb to gamma delta TCR, together with rabbit complement, or by panning on anti-hamster Ig-coated dishes, diminished substantially the subsequent transfer of CS reactivity without affecting either CS-initiating cells, or the later-acting, alpha beta TCR+ CS effector T cells. Immune cells treated with anti-alpha beta TCR mAb, or recovered as adherent cells from petri dishes after anti-gamma delta TCR panning (i.e., gamma delta TCR-enriched cells), reconstituted the ability of anti-gamma delta TCR-treated immune cells (i.e., alpha beta TCR-enriched cells) to transfer 24-h CS responsiveness. The phenotype of the gamma delta T cells that assisted CS effector alpha beta T cells was: CD3+, CD4-, and CD8+. The gamma delta T cells that assisted alpha beta T cells were not Ag-specific since anti-alpha beta-TCR-treated cells (gamma delta T-enriched) from picryl chloride immunized donors aided alpha beta T cells (anti-gamma delta TCR-treated) from oxazolone-immunized donors, and conversely gamma delta T cells from oxazolone-immunized donors aided alpha beta T cells from picryl chloride immunized donors. Furthermore, the CS-regulating gamma delta T cells were not MHC-restricted because gamma delta T cells from H2d or H2b donors could assist alpha beta T cells from H2k donors. It was concluded that a regulatory population of non-Ag specific, non-MHC-restricted gamma delta T cells was needed to assist immune effector, Ag/MHC-specific alpha beta T cells in the adoptive transfer of CS.     

A purified parasite antigen (p30) mediates CD8+ T cell immunity against fatal Toxoplasma gondii infection in mice

Induction of protective immunity against acute and chronic toxoplasmosis can be achieved using p30, the major membrane and excreted/secreted protein of Toxoplasma gondii. This protein, when administered to outbred mice in the presence of the saponin Quil A, is able to induce almost 100% protection against acute infection without evidence of intracerebral cyst development. Adoptive transfer of immune splenocytes from immunized inbred A/J mice conferred a significant level (p less than 0.001) of protection against subsequent challenge. Phenotypic analysis in outbred as well as two different strains of inbred mice (A/J and C57BL/6) demonstrated that CD8+ T cells are selectively stimulated by this immunization protocol. T cell depletion studies using specific mAb directed at either CD3+ or CD8+ T cell phenotype, followed by adoptive transfer, failed to confer protective immunity, whereas CD4+ depletion had no effect. These cytotoxic CD8+ T cells produced high titers of both IFN-gamma and IL-2. Moreover, these CD8+ T cells were directly parasiticidal against radiolabeled extracellular T. gondii, further supporting the critical immune function of these p30 Ag-specific CD8+ T cells in host immunity against T. gondii infection.     

Differential kinetics of antigen-specific CD4+ and CD8+ T cell responses in the regression of retrovirus-induced sarcomas

Despite the accepted role for CD4+ T cells in immune control, little is known about the development of Ag-specific CD4+ T cell immunity upon primary infection. Here we use MHC class II tetramer technology to directly visualize the Ag-specific CD4+ T cell response upon infection of mice with Moloney murine sarcoma and leukemia virus complex (MoMSV). Significant numbers of Ag-specific CD4+ T cells are detected both in lymphoid organs and in retrovirus-induced lesions early during infection, and they express the 1B11-reactive activation-induced isoform of CD43 that was recently shown to define effector CD8+ T cell populations. Comparison of the kinetics of the MoMSV-specific CD4+ and CD8+ T cell responses reveals a pronounced shift toward CD8+ T cell immunity at the site of MoMSV infection during progression of the immune response. Consistent with an important early role of Ag-specific CD4+ T cell immunity during MoMSV infection, CD4+ T cells contribute to the generation of virus-specific CD8+ T cell immunity within the lymphoid organs and are required to promote an inflammatory environment within the virus-infected tissue.     

Adoptive transfer of CD8+ T cells from immune animals does not transfer immunity to blood stage Plasmodium yoelii malaria

The malaria parasite, Plasmodium yoelii 17X, causes a self-limited, nonlethal infection characterized, in the blood stage, by preferential invasion of reticulocytes. Previous studies have suggested that immunity to the blood stage infection may be related to enhanced levels of class I MHC Ag on the parasitized reticulocyte surface and can be adoptively transferred to immunodeficient mice by immune CD8+ T cells in the absence of CD4+ T cells. To further examine the mechanisms of CD8+ T cell involvement in immunity to blood stage P. yoelii infection, we performed in vivo CD8 depletion and adoptive transfer experiments. Depletion of CD8+ T cells during primary blood stage infection in BALB/c mice did not diminish the ability of the mice to resolve their infections. Spleen cells from immune BALB/c and C57BL/10 mice were transferred to BALB/c-nu/nu and C57BL/10-nu/nu mice, respectively. The recipient mice were CD4 depleted in vivo to kill any transferred CD4+ T cells. The mice failed to control the infection. Populations of CD4-, CD8+ T cells were transferred from immune CBA/CaJ donors to in vivo CD4-depleted CBA/CaJ recipients. The mice were unable to control the infection. Although immune unfractionated spleen cells transferred rapid protection in all three mouse strains and immune CD4+ T cells transferred immunity in the two mouse strains studied, CD8+ T cells by themselves were neither protective nor did they enhance immunity.     

IL-10 is required for optimal CD8 T cell memory following Listeria monocytogenes infection

IL-10 is an important immunoregulatory cytokine that plays a central role in maintaining a balance between protective immunity against infection and limiting proinflammatory responses to self or cross-reactive Ags. We examined the full effects of IL-10 deficiency on the establishment and quality of T cell memory using murine listeriosis as a model system. IL-10(-/-) mice had reduced bacterial loads and a shorter duration of primary infection than did wild-type mice. However, the number of Ag-specific T cells in secondary lymphoid and nonlymphoid organs was diminished in IL-10(-/-) mice, compared with wild-type mice, at the peak of the effector response. Moreover, the frequency and protective capacity of memory T cells also were reduced in IL-10(-/-) mice when assessed up to 100 days postinfection. Remarkably, this effect was more pronounced for CD8 T cells than CD4 T cells. To address whether differences in the number of bacteria and duration of primary infection could explain these findings, both strains of mice were treated with ampicillin 24 hours after primary infection. Despite there being more comparable bacterial loads during primary infection, IL-10(-/-) mice still generated fewer memory CD8 T cells and were less protected against secondary infection than were wild-type mice. Finally, the adoptive transfer of purified CD8 T cells from previously infected wild-type mice into naive recipients conferred better protection than the transfer of CD8 T cells from immune IL-10(-/-) mice. Overall, these data show that IL-10 plays an unexpected role in promoting and/or sustaining CD8 T cell memory following Listeria monocytogenes infection.     

CD28 is required for T cell activation and IFN-gamma production by CD4+ and CD8+ T cells in response to Trypanosoma cruzi infection

In the present study we evaluated the mechanisms behind the implication of the costimulatory molecule CD28 for the immune response against the intracellular protozoan parasite Trypanosma cruzi. Our results reveal a critical role for CD28 in the activation of both CD4+ and CD8+ T cells and induction of the effector mechanisms that ultimately mediate the control of parasite growth and pathogenesis in infected mice. CD28-deficient (CD28-/-) mice are highly susceptible to T. cruzi infection, presenting higher parasitemia and tissue parasitism, but less inflammatory cell infiltrate in the heart than C57Bl/6 wild-type (WT) mice. All the infected WT mice survived acute infection, whereas 100% of CD28-/- mice succumbed to it. The increased susceptibility of the CD28-/- mice was associated with a dramatic decrease in the production of IFN-gamma by both CD4+ and CD8+ T cells resulting in a diminished capacity to produce nitric oxide (NO) and mediate parasite killing. T cell activation was also profoundly impaired in CD28-/- mice, which presented decreased lymphoproliferative response after the infection compared to WT mice. Together, these data represent the first evidence that CD28 is critical for efficient CD4+ T cell activation in response to T. cruzi infection in mice.     

Sendai virus pneumonia: evidence for the early recruitment of gamma delta T cells during the disease course.

We previously reported that gamma delta T cells appeared and could play a protective role early in infections with intracellular bacteria such as Listeria monocytogenes, Mycobacterium bovis BCG, and Salmonella choleraesuis. To extend these findings to virus infection, we examined the developmental sequence of gamma delta T cells in bronchoalveolar lavage during the course of Sendai virus infection in C57BL/6 mice. To produce a natural but nonlethal infection course as far as possible, we used a sublethal dose of a wild-type virus which had not been subjected to serial passages in a chicken embryo, hence retaining full virulence for mice. Virus titers in lungs reached a peak on day 6 and then decreased to an undetectable level by day 10. This time course of virus reproduction was immediately and coincidentally followed by the developmental course of gamma delta T cells, in which the cell number peaked on day 7 and then decreased to a marginal level by day 10. On the other hand, the alpha beta T-cell number continued to increase until day 10 and remained at a high level thereafter. The early-appearing gamma delta T cells were CD4-, CD8-, IL-2R alpha- beta+, CD44+, Mel-14-, and LFA-1 alpha/beta+ in phenotype and used V gamma 1/2 and V gamma 4 and V delta 3, V delta 4, V delta 5, and V delta 6. The gamma delta T cells were responding to macrophages from infected mice when the cells were cultured in vitro. Furthermore, the expression of endogenous heat shock protein (hsp) was infection specific, and its level appeared to correlate with the gamma delta T-cell development. These results suggest that the early recruitment of gamma delta T cells, which proliferate in response to endogenous hsp+ cells, is also characteristic of this virus infection, although this view appears to be contradictory to earlier reports.     

Protective cellular retroviral immunity requires both CD4+ and CD8+ immune T cells.

We have found previously that postexposure chemoprophylaxis with 3'-azido-3'-deoxythymidine (also known as zidovudine or AZT) in combination with recombinant human alpha A/D interferon fully protected mice exposed to a lethal dose of Rauscher murine leukemia virus (RLV) against viremia and disease. After cessation of therapy, over 90% of these mice were able to resist rechallenge with live RLV, thus demonstrating an acquired immunity. Adoptive cell transfer of 4 x 10(7) cells from immunized mice fully protected naive recipients from viremia and splenomegaly after RLV challenge. However, when these immune T cells were fractionated into CD4+ and CD8+ subpopulations, only partial protection was found when 4 x 10(7) T cells of either subset were given. Full protection against RLV challenge was seen again when the T-cell subsets from immunized mice were recombined and transferred at the same number into naive mice. We conclude that cellular immunity alone is protective and that both CD4+ and CD8+ cell types are required for conferring full protection against live virus challenge.