Plasmodium chabaudi chabaudi (AS): differential cellular responses to infection in resistant and susceptible mice

The infection with blood stages of Plasmodium chabaudi chabaudi (AS) was followed in BALB/c and DBA/2 mice. Both strains show a peak parasitemia by 7-9 days after infection, display splenic hypercellularity of T and B cells, thymic atrophy, nearly complete depletion of B cells in the bone marrow, and mount comparable polyclonal IgM and IgG responses in the serum. In contrast, these strains diverge in some aspects of the immune response and susceptibility to infection: while BALB/c survive, 70-80% of DBA/2 die within 2 weeks; BALB/c but not DBA/2 show marked increases in the levels of splenic gamma/delta and regulatory T cells, dendritic cells and macrophages and parasite-specific IgM and IgG levels; however, lower levels of TNF-alpha and IL-12 were observed. These results suggest the relevance of different cell populations that are known to participate/regulate specific antibody responses and cytokine production in the susceptibility to infection.     

Plasmodium chabaudi chabaudi infection in mice induces strong B cell responses and striking but temporary changes in splenic cell distribution

B cells and Abs play a key role in controlling the erythrocytic stage of malaria. However, little is known about the way the humoral response develops during infection. We show that Plasmodium chabaudi chabaudi causes major, but temporary changes in the distribution of leukocytes in the spleen. Despite these changes, an ordered response to infection develops, which includes vigorous extrafollicular growth of plasmablasts and germinal center formation. Early in the response, the lymphocytes in the T zone and follicles become widely spaced, and the edges of these compartments blur. This effect is maximal around the peak of parasitemia. Germinal centers are apparent by day 8, peak at day 20, and persist through day 60. Extrafollicular foci of plasmablasts are visible from day 4 and initiate a very strong plasma cell response. Initially, the plasma cells have a conventional red pulp distribution, but by day 10 they are unconventionally sited in the periarteriolar region of the white pulp. In this region they form clusters occupying part of the area normally filled by T cells. B cells are absent from the marginal zone for at least 30 days after the peak of infection, although flow cytometry shows their continued presence in the spleen throughout infection. Relatively normal splenic architecture is regained by day 60 of infection. These results show that the changes in splenic cell distribution are linked to the presence of parasites and do not seem to interfere with the development of the humoral response.     

Distinct kinetics of memory B-cell and plasma-cell responses in peripheral blood following a blood-stage Plasmodium chabaudi infection in mice

B cell and plasma cell responses take place in lymphoid organs, but because of the inaccessibility of these organs, analyses of human responses are largely performed using peripheral blood mononuclear cells (PBMC). To determine whether PBMC are a useful source of memory B cells and plasma cells in malaria, and whether they reflect Plasmodium-specific B cell responses in spleen or bone marrow, we have investigated these components of the humoral response in PBMC using a model of Plasmodium chabaudi blood-stage infections in C57BL/6 mice. We detected memory B cells, defined as isotype-switched IgD(-) IgM(-) CD19(+) B cells, and low numbers of Plasmodium chabaudi Merozoite Surface Protein-1 (MSP1)-specific memory B cells, in PBMC at all time points sampled for up to 90 days following primary or secondary infection. By contrast, we only detected CD138(+) plasma cells and MSP1-specific antibody-secreting cells within a narrow time frame following primary (days 10 to 25) or secondary (day 10) infection. CD138(+) plasma cells in PBMC at these times expressed CD19, B220 and MHC class II, suggesting that they were not dislodged bone-marrow long-lived plasma cells, but newly differentiated migratory plasmablasts migrating to the bone marrow; thus reflective of an ongoing or developing immune response. Our data indicates that PBMC can be a useful source for malaria-specific memory B cells and plasma cells, but extrapolation of the results to human malaria infections suggests that timing of sampling, particularly for plasma cells, may be critical. Studies should therefore include multiple sampling points, and at times of infection/immunisation when the B-cell phenotypes of interest are likely to be found in peripheral blood.     

Plasmodium chabaudi chabaudi: B-1 cell expansion correlates with semiresistance in BALB/cJ mice

The largest obstacle impeding the development of an effective malaria vaccine is the incomplete understanding of how the immune response is regulated during infection. B-1a cells, a poorly understood subcategory of B lymphocytes, produce nonpathologic autoantibodies of low affinity which have been shown to have distinct immunoregulatory capabilities. What the exact activity of B-1a cells are during the course of malaria has yet to be determined. By use of flow cytometry, it was observed that B-1a cells significantly expand by day 3 postinfection in the spleen and peritoneum of Plasmodium chabaudi chabaudi semiresistant BALB/cJ mice, but not until day 8 postinfection in the spleen of P. chabaudi chabaudi fully susceptible BALB/cByJ mice. The activation of B-1a cells was also demonstrated by the measurement of natural autoantibody IgM production from the serum and cultured peritoneal B-1a cells. Infected semiresistant BALB/cJ mice generated higher levels of anti-ssDNA IgM antibodies than infected fully susceptible BALB/cByJ mice. The preliminary data presented here suggest a possible roll of B-1 cells in contributing to the successful survival of murine malarial infection.     

致死型夏氏疟原虫感染BALB/c小鼠CD4~＋ CD25~＋ Foxp3~＋调节性T细胞在Th2免疫应答极化中的作用研究

为探讨CD4＋ CD25＋ Foxp3＋调节性T细胞（Treg细胞）在疟疾感染过程中对Th2极化的调控作用,利用Treg细胞消除的致死型夏氏疟原虫（Plasmodium chabaudi chabaudi AS,P.c chabaudi AS）感染鼠疟模型进行研究。结果显示,对照组小鼠在感染后8 d原虫血症达到峰值40.5%,随后迅速下降,于感染后18 d小鼠自愈。相比,Treg细胞消除组于感染后10 d,原虫血症水平迅速上升至32%,随后小鼠相继死亡。在感染后8～10 d,Treg细胞消除小鼠脾脏CD4＋ CD25＋ Foxp3＋细胞占CD4＋细胞百分比含量明显低于对照组。同时,血清疟原虫特异性抗体IgG1和IgG2a水平均明显降低。结果提示,P.c chabaudi AS感染中CD4＋ CD25＋ Foxp3＋细胞参与调控Th2型免疫应答的极化,进而干预疟原虫清除。     

The spleen CD4+ T cell response to blood-stage Plasmodium chabaudi malaria develops in two phases characterized by different properties

The pivotal role of spleen CD4(+) T cells in the development of both malaria pathogenesis and protective immunity makes necessary a profound comprehension of the mechanisms involved in their activation and regulation during Plasmodium infection. Herein, we examined in detail the behaviour of non-conventional and conventional splenic CD4(+) T cells during P. chabaudi malaria. We took advantage of the fact that a great proportion of CD4(+) T cells generated in CD1d(-/-) mice are I-A(b)-restricted (conventional cells), while their counterparts in I-A(b-/-) mice are restricted by CD1d and other class IB major histocompatibility complex (MHC) molecules (non-conventional cells). We found that conventional CD4(+) T cells are the main protagonists of the immune response to infection, which develops in two consecutive phases concomitant with acute and chronic parasitaemias. The early phase of the conventional CD4(+) T cell response is intense and short lasting, rapidly providing large amounts of proinflammatory cytokines and helping follicular and marginal zone B cells to secrete polyclonal immunoglobulin. Both TNF-α and IFN-γ production depend mostly on conventional CD4(+) T cells. IFN-γ is produced simultaneously by non-conventional and conventional CD4(+) T cells. The early phase of the response finishes after a week of infection, with the elimination of a large proportion of CD4(+) T cells, which then gives opportunity to the development of acquired immunity. Unexpectedly, the major contribution of CD1d-restricted CD4(+) T cells occurs at the beginning of the second phase of the response, but not earlier, helping both IFN-γ and parasite-specific antibody production. We concluded that conventional CD4(+) T cells have a central role from the onset of P. chabaudi malaria, acting in parallel with non-conventional CD4(+) T cells as a link between innate and acquired immunity. This study contributes to the understanding of malaria immunology and opens a perspective for future studies designed to decipher the molecular mechanisms behind immune responses to Plasmodium infection.     

Antigen-specific, interleukin 2-propagated T lymphocytes confer resistance to a murine malarial parasite, Plasmodium chabaudi adami

To explore cell-mediated immune mechanisms in host defense against malaria, we utilized a murine model system in which antibody-independent mechanisms of immunity are known to play a major role. Splenic T lymphocytes obtained from Plasmodium chabaudi adami-immune mice were maintained in vitro by using IL 2-containing medium and frequent antigenic stimulation. These IL 2-propagated T lymphocytes were characterized for their antigen reactivity, surface phenotype, and ability to confer protection to P. chabaudi adami in reconstituted mice. IL 2-dependent T lymphocytes maintained their capacity to proliferate in vitro to solubilized parasite preparations of homologous but not heterologous antigens. Antigen-specific proliferation was H-2 restricted, requiring antigen-presenting cells of the correct haplotype. More importantly, these propagated T lymphocytes were effective in adoptively transferring protection to both athymic nude mice and sublethally irradiated recipients. The protective response was dose dependent and antigen specific, because recipients resisted challenge infection with P. chabaudi adami but not with the heterologous parasite Plasmodium yoelii 17X. Pretreatment of the IL 2-propagated cells with anti-Thy-1.2 and complement abrogated their ability to transfer protection. Collectively, these results suggest that T lymphocytes obtained from P. chabaudi adami-immune mice, propagated and expanded in vitro, retain antigen specificity and passive protective activity in vivo.     

CD8+ T lymphocyte-mediated loss of marginal metallophilic macrophages following infection with Plasmodium chabaudi chabaudi AS

The splenic architecture is essential for the quick resolution of a primary infection with Plasmodium. A critical component of this architecture is the marginal zone (MZ), an area of the spleen that separates the reticuloendothelial red pulp of the spleen from the lymphoid white pulp compartment. There are two unique macrophage populations found in the MZ: MZ macrophages (MZM) found on the outer border of the MZ, and marginal metallophilic macrophages (MMM) found on the inner border, adjacent to the white pulp. We investigated the homeostasis of MMM and MZM following infection with Plasmodium chabaudi and demonstrated that a complete loss of both MMM and MZM occurred by the time of peak parasitemia, 8 days after infection. The loss was not induced by up-regulation of the inflammatory cytokines TNF or IFN-gamma. In contrast, following only CD8+ T cell depletion (not dendritic cell), MMM but not MZM were retained, implicating CD8+ T cells in the P. chabaudi-induced loss of MMM. Retention of MMM occurred in mice deficient in CD95, CD95-ligand, and perforin, indicating that these signals are involved in the death pathway of MMM. These data have significant implications for the understanding of the immune-mediated pathology of the spleen as a result of infection with Plasmodium.     

夏氏疟原虫感染过程中调节性B细胞的表达变化

调节性B细胞(regulatory B cells,Bregs)是近年来发现的通过分泌IL-10发挥免疫调节作用的B细胞,其在疟疾免疫应答中的作用尚不清楚。利用血液阶段夏氏疟原虫(Plasmodium chabaudi AS,P.c AS)感染的BALB/c小鼠,观察了感染过程中Bregs数量变化及其表面分子表达情况。结果显示,BALB/c小鼠感染P.c AS后红细胞感染率逐渐升高,于感染后第9天达到30%,多数小鼠死亡。脾组织细胞因子IL-10 mRNA水平在感染后显著升高,感染后第5天达到高峰,感染后第8天仍为正常小鼠的10倍左右。CD4+细胞中IL-10+细胞百分比和绝对数量在感染后第5天达到峰值,于感染后第8天回落至正常水平;而CD19+细胞中IL-10+细胞(Bregs)百分比在感染后持续上升,在感染后第8天达到CD19+细胞的5%左右;感染后第5天,脾中CD4+IL-10+细胞绝对数量高于CD19+IL-10+细胞,至感染后第8天,CD19+IL-10+细胞绝对数量显著高于CD4+IL-10+细胞(P﹤0.01),约90%Bregs为CD5-细胞。结果提示,P.c AS感染过程中Bregs显著活化,是感染1周后IL-10的主要产生细胞。     

Immune complexes and immunoconglutinin interactions associated with altered lymphocyte activity in Plasmodium chabaudi infections

Fresh plasma from rats infected with Plasmodium chabaudi, incubated with splenic lymphocytes from rats immunized 5 days previously with sheep blood cells, suppressed the capacity of the spleen cells to produce antibody against the sheep cells as was indicated by reductions in the numbers of hemolytic Jerne plaques formed by the treated cells. The effect was maximal in plasma of rats drawn on the 7th day of infection at a time the rats experienced a hemolytic crisis. Serologic studies indicated that the active plasma contained elevated titers of antibody against fibrinogen products, antibody against the soluble serum antigens elaborated during blood infections and antibody against the third component of fixed complement (C3) or immunoconglutinin. Titers of lytic complement were reduced and amounts of soluble immune complex precipitated with polyethylene glycol 6000 were elevated. The active plasma may have affected the antibody producing cells by one or both of two mechanisms. Soluble antigen-antibody complexes could have interacted with Fc receptors of activated lymphocytes to alter their function. Alternatively, the complexes may have fixed complement and interacted with receptors for fixed C3 on the lymphocyte membrane. Such cells, being coated with the antigen for immunoconglutinin, could be altered by immunoconglutination. Inasmuch as the immune complexes in the active plasma were generated in vivo, it would seem unlikely that the plasma would contain significant amounts of complex that had not fixed complement. With immunoconglutinin present in the plasma, alteration of the cells by immunoconglutination seems a more likely possibility.     

Adoptive protection against Plasmodium chabaudi adami malaria in athymic nude mice by a cloned T cell line

T cell-dependent, cell-mediated immune mechanisms have been shown to contribute to resistance against malaria. Because the identity of plasmodial Ag responsible for the activation of these protective immune responses remains unknown, a major step in isolating these potential immunizing agents will be the development of adequate screening procedures designed to identify important T cell Ag. This study focused on the isolation of protective T cell clones that may play a pivotal role in this process. A T cell clone designated CTR2.1 and two subclones derived from it adoptively transferred protection to athymic nude mice infected with Plasmodium chabaudi adami, a murine malarial parasite known to be recognized by protective thymus-dependent immune mechanisms. The protective T cell clone displayed a L3T4+, Lyt-2- surface phenotype and secreted both IFN-gamma and IL-2 after stimulation with solubilized parasites in vitro. This is the first report of results demonstrating a cloned T cell line capable of providing adoptive protection against malaria in vivo. More importantly, CTR2.1 and other protective T cell clones may provide for the identification of plasmodial antigenic epitopes recognized by important cell-mediated immune mechanisms during acute malarial infection.     

Recombinant viral-vectored vaccines expressing Plasmodium chabaudi AS apical membrane antigen 1: mechanisms of vaccine-induced blood-stage protection

Apical membrane Ag 1 (AMA1) is one of the leading candidate Ags for inclusion in a subunit vaccine against blood-stage malaria. However, the efficacy of Ab-inducing recombinant AMA1 protein vaccines in phase IIa/b clinical trials remains disappointing. In this article, we describe the development of recombinant human adenovirus serotype 5 and modified vaccinia virus Ankara vectors encoding AMA1 from the Plasmodium chabaudi chabaudi strain AS. These vectors, when used in a heterologous prime-boost regimen in BALB/c mice, are capable of inducing strong transgene-specific humoral and cellular immune responses. We show that this vaccination regimen is protective against a nonlethal P. chabaudi chabaudi strain AS blood-stage challenge, resulting in reduced peak parasitemias. The role of vaccine-induced, AMA1-specific Abs and T cells in mediating the antiparasite effect was investigated by in vivo depletion of CD4(+) T cells and adoptive-transfer studies into naive and immunodeficient mice. Depletion of CD4(+) T cells led to a loss of vaccine-induced protection. Adoptive-transfer studies confirmed that efficacy is mediated by both CD4(+) T cells and Abs functioning in the context of an intact immune system. Unlike previous studies, these results confirm that Ag-specific CD4(+) T cells, induced by a clinically relevant vaccine-delivery platform, can make a significant contribution to vaccine blood-stage efficacy in the P. chabaudi model. Given that cell-mediated immunity may also contribute to parasite control in human malaria, these data support the clinical development of viral-vectored vaccines that induce both T cell and Abs against Plasmodium falciparum blood-stage malaria Ags like AMA1.     

Natural regulatory (CD4+CD25+FOXP+) T cells control the production of pro-inflammatory cytokines during Plasmodium chabaudi adami infection and do not contribute to immune evasion

Different functions have been attributed to natural regulatory CD4+CD25+FOXP+ (Treg) cells during malaria infection. Herein, we assessed the role for Treg cells during infections with lethal (DS) and non-lethal (DK) Plasmodium chabaudi adami parasites, comparing the levels of parasitemia, inflammation and anaemia. Independent of parasite virulence, the population of splenic Treg cells expanded during infection, and the absolute numbers of activated CD69+ Treg cells were higher in DS-infected mice. In vivo depletion of CD25+ T cells, which eliminated 80% of CD4+FOXP3+CD25+ T cells and 60-70% of CD4+FOXP3+ T cells, significantly decreased the number of CD69+ Treg cells in mice with lethal malaria. As a result, higher parasite burden and morbidity were measured in the latter, whereas the kinetics of infection with non-lethal parasites remained unaffected. In the absence of Treg cells, parasite-specific IFN-gamma responses by CD4+ T cells increased significantly, both in mice with lethal and non-lethal infections, whereas IL-2 production was only stimulated in mice with non-lethal malaria. Following the depletion of CD25+ T cells, the production of IL-10 by CD90(-) cells was also enhanced in infected mice. Interestingly, a potent induction of TNF-alpha and IFN-gamma production by CD4+ and CD90(-) lymphocytes was measured in DS-infected mice, which also suffered severe anaemia earlier than non-depleted infected controls. Taken together, our data suggest that the expansion and activation of natural Treg cells represent a counter-regulatory response to the overwhelming inflammation associated with lethal P.c. adami. This response to infection involves TH1 lymphocytes as well as cells from the innate immune system.     

Lymphoid cell kinetics in guinea pigs infected with Trichostrongylus colubriformis: tritiated thymidine uptake in gut and allied lymphoid tissue, humoral IgE and hemagglutinating antibody responses, delayed hypersensitivity reactions, and in vitro lymphocyte transformations during primary infections

The kinetics of the lymphocyte responses of Trichostrongylus colubriformis-infected and normal guinea pigs were measured by the in vivo uptake of tritiated thymidine either as dpm ³H/mg tissue or as the percentage change in [³H] -labeled lymphoblasts in autoradiographs of tissue impression smears and sections. The lymphoid response was predominantly a local one centering on the infected area of the small intestine. The greatest lymphocyte reactions as assessed by counts of labeled lymphoblasts occurred in the Peyer's patches and mesenteric lymph nodes where the peak responses took place 11 and 6 days after infection, respectively. The local nature of the responses was exemplified by the fact that the mesenteric lymph nodes of the anterior small intestine showed a peak response on the sixth day but the response from the posterior small intestine peaked 7 days later. A similar but less dramatic relationship existed among the Peyer's patches. In addition no labeled lymphoblast response was elicited in the inguinal lymph nodes or cecal lymphoid patches throughout the infection and the first increased responsiveness of the spleen did not take place until after Day 13, by which time the lymphoid proliferations associated with the infected intestine had subsided. Initially, the spleen showed a marked depletion of labeled blast cells during the first 7 days of the infection. This was taken as indicating at the time the infection was being established the export of cells capable of transformation in response to parasite antigen. This was supported by the observation that large numbers of phytohemagglutinin responsive lymphocytes were found in the peripheral circulation at this time. The in vitro responsiveness of peripheral lymphocytes to T. colubriformis antigen was also studied. Positive lymphocyte transformations first occurred 6 days after infection but thereafter declined to the normal level by Day 13; the peak transformation ratio was found 25 days after infection but by Day 38 it had declined to a low but persistently positive level. There was a correlation between the circulation of specifically sensitized cells, probably of thymic origin, IgE antibody titers, and the development of positive dermal delayed hypersensitivity reactions in infected guinea pigs, suggesting a close relationship among these three immunological phenomena.All lymphoblast responses in Peyer's patches, mesenteric lymph nodes, and lamina propria of the intestine were completed before the immune elimination of the parasite commenced 10 days after infection. During the first 10 days of infection specifically sensitized lymphocytes appeared and disappeared from the circulation. The loss of circulating sensitized lymphocytes at the time immune elimination of the parasite was taking place in the gut suggested that the sensitized cells were “homing-in” on the local area of infection. After the immune elimination of the parasite had commenced, the level of sensitized lymphocytes and IgE antibodies then increased rapidly in the blood. Evidence from the kinetics of the hemagglutinating antibodies indicated that stage specific antigens occur in T. colubriformis.     

NK cells and innate immunity to malaria

Innate immune response against Plasmodium falciparum (Pf), a causative agent of human malaria, is the result of several thousand years of co-evolution between the parasite and his host. An early IFN-gamma production during infection is associated with a better evolution of the disease. Natural killer (NK) cells are among the first cells in peripheral blood to produce IFN-gamma in response to Pf-infected erythrocytes (Pf-E). NK cells are found in blood, in secondary lymphoid organs as well as in peripheral non-lymphoid tissues. They participate in host innate responses that occur upon viral and intracytoplasmic bacterial infections, but also during the course of tumor development and allogeneic transplantation. These lymphocytes are not only important players of innate effector responses, but also participate in the initiation and development of adaptive immune responses. In addition, direct sensing of Pf infection by NK cells induces their production of the proinflammatory chemokine IL-8, suggesting a role for NK cells in the recruitment and the activation of other cells during malaria infection. Several other cell subsets are involved in the innate immune response to Pf. Dendritic cells, macrophages, gamma delta T cells, NKT cells are able to sense the presence of the parasite. Along this line, the presence of IL-12 is necessary to NK cell IFN-gamma production and a functional cooperation takes place between macrophages and NK cells in the context of this parasitic infection. In particular, IL-18 produced by macrophages is a key factor for this NK response. However, the molecular basis of Pf-E recognition by NK cells as well as the functional role of NK cell responses during the course of the disease remain to be adressed.     

Possible modulation by male sex hormone of Th1/Th2 function in protection against Plasmodium chabaudi chabaudi AS infection in mice

Zhang, Z.-H., Chen, L., Saito, S., Kanagawa, O., and Sendo, F. 2000. Possible modulation by male sex hormone of Th1/Th2 function in protection against Plasmodium chabaudi chabaudi AS infection in mice. Experimental Parasitology 96, 121-129. We examined the mortality, survival time, and parasitemia in interferon gamma receptor (IFN-gamma R)-deficient (IFN-gamma R(-/-)) and IL-4-deficient (IL-4(-/-)) mice infected with Plasmodium chabaudi AS and compared them with the wild type counterparts (IFN-gamma R(+/+) and IL-4(+/+), respectively). (1) Mortality was higher and survival time was shorter in males of both IFN-gamma R(-/-) and IL-4(-/-) mice infected with P. chabaudi AS, compared with their wild type counterparts, whereas such a difference was not observed in female mice. (2) These differences between males and females were not observed when male mice were castrated; however, female castration had no effect on the data. (3) The rate of parasitemia in both male and female IFN-gamma R(-/-) and IL-4(-/-) mice was higher at some points during the observation than in the wild type counterparts. (4) These results on susceptibility vs resistance to P. chabaudi AS infection can be explained partially by the levels of expression of Th1/Th2 cytokine and chemokine mRNAs in the spleen cells of the infected mice. These results suggest that male sex hormones modulate the function of Th1/Th2 cells and that these T cells counteract the activity of these hormones in protection against P. chabaudi AS infection in mice.     

CD4+CD25+Foxp3hi细胞在夏氏疟原虫感染DBA/2小鼠作用研究

利用调节性T细胞消除的致死型夏氏疟原虫(Plasmodium chabaudi chabaudi AS,P.c chabaudi AS)感染鼠疟模型,探讨DBA/2小鼠对P.c chabaudi AS感染易感性的原因。DBA/2小鼠对P.c chabaudi AS易感,伴随原虫血症增加CD4+CD25+Foxp3+细胞数量明显增加,且以CD4+CD25+Foxp3hi增加更为明显。原虫血症达峰值时CD4+CD25+Foxp3hi细胞数量亦达到峰值。相比,Treg消除鼠的原虫出现时间和疟血症峰值时间均明显延迟,且在疟血症达峰值前(5～8 d)原虫血症水平明显低于对照组。与之相应,CD4+CD25+Foxp3hi细胞数量明显处于低水平。同时,Treg消除鼠生存期明显延长。由此提示,P.c chabaudi AS感染导致Foxp3表达增加,扩增的CD4+CD25+Foxp3hi细胞有利于疟原虫复制和逃避宿主免疫应答,进而影响疟疾感染的进程和最终结局。     

Natural killer cells vs cytotoxic T cells in the peripheral blood of virus-infected mice

The cell-mediated immune response of mice against various enveloped RNA and DNA viruses expressed by immune lymphocytes from the spleen and the peripheral blood (PBL) were compared. PBL from mice of various strains infected with vaccinia virus, vesicular stomatitis virus (VSV), or lymphocytic choriomeningitis virus (LCMV) were tested on histocompatible or incompatible target cells infected with the homologous virus. PBL from immune mice showed clear H-2 restriction, but additionally, they expressed high natural killing (NK) activity on YAC-1 cells. The high NK-cytolytic activity of PBL on YAC-1 differed significantly from that expressed by splenic lymphocytes. In both lymphocyte populations lysis was detected as early as 1 day after infection; NK activity decreased in the spleen after day 4 post infection, whereas that of PBL persisted at high levels for up to 10 days after infection. Treatment of mice with anti-asialo GM1 in vivo abrogated NK activity in PBL effector cells tested in vitro. These results may explain some of the difficulties to observe MHC-restricted cytotoxic T cells in PBL from humans or primates during primary infections with virus.