Abrogation of anti-Pichinde virus cytotoxic T cell memory by cyclophosphamide and restoration by coinfection or interleukin 2

Previously, we demonstrated that memory cell-mediated immune responses can be generated in Pichinde virus (PV)-primed mice after secondary challenge in vivo with homologous virus. Further, treatment of mice with cyclophosphamide (CY) before primary infection with PV abrogated the generation of H-2-restricted, virus-specific cytotoxic T lymphocytes (CTL), and rechallenge of these mice was followed by neither a primary nor a secondary CTL response. Here, we demonstrate that this CY-induced block in memory anti-PV CTL generation was not due to establishment of a persistent infection. Interestingly, this CY-induced block in memory anti-PV CTL generation was overcome by secondarily coinfecting mice with PV and lymphocytic choriomeningitis virus (LCMV) or PV and Tacaribe virus. Secondary infection with LCMV or Tacaribe virus alone did not elicit anti-PV CTL. Coinfection resulted in the generation of a PV-specific memory CTL response as judged by maximal activity on day 4 after rechallenge. Co-infection with PV and vesicular stomatitis virus, an unrelated rhabdovirus, did not efficiently restore memory anti-PV CTL responses. Memory anti-PV CTL responses were also restored when interleukin 2 (IL 2)-containing supernatants were injected i.p. after rechallenge of CY-treated mice with PV. To demonstrate that IL 2 was the responsible lymphokine in these preparations, highly purified IL 2 was added to in vitro cultures of spleen cells from CY-treated PV-primed mice. In the presence of PV-infected syngeneic macrophages, addition of purified IL 2 resulted in a dose-dependent restoration of H-2-restricted anti-PV CTL activity. The CTL precursor (CTLp) frequency of CY-treated PV-primed mice was markedly decreased from that of normal PV-primed mice. Thus, the long-lasting block in the ability to generate a PV-specific memory CTL response after CY treatment appears to be due to both a lack of helper T cell activity and a significant reduction of CTLp. However, this block may be overcome by coinfecting with viruses that cross-react at the helper T cell level or by exogenous treatment with highly purified IL 2.     

Expression of mouse interleukin-4 by a recombinant ectromelia virus suppresses cytolytic lymphocyte responses and overcomes genetic resistance to mousepox

Genetic resistance to clinical mousepox (ectromelia virus) varies among inbred laboratory mice and is characterized by an effective natural killer (NK) response and the early onset of a strong CD8(+) cytotoxic T-lymphocyte (CTL) response in resistant mice. We have investigated the influence of virus-expressed mouse interleukin-4 (IL-4) on the cell-mediated response during infection. It was observed that expression of IL-4 by a thymidine kinase-positive ectromelia virus suppressed cytolytic responses of NK and CTL and the expression of gamma interferon by the latter. Genetically resistant mice infected with the IL-4-expressing virus developed symptoms of acute mousepox accompanied by high mortality, similar to the disease seen when genetically sensitive mice are infected with the virulent Moscow strain. Strikingly, infection of recently immunized genetically resistant mice with the virus expressing IL-4 also resulted in significant mortality due to fulminant mousepox. These data therefore suggest that virus-encoded IL-4 not only suppresses primary antiviral cell-mediated immune responses but also can inhibit the expression of immune memory responses.     

Rapid conversion of effector mechanisms from NK to T cells during virus-induced lysis of allogeneic implants in vivo

Viral infections can strongly stimulate both NK cell and allospecific CD8 T cell responses, and these same effector cells can lyse allogeneic cell lines in vitro. However, the impact of viral infections on the effector systems mediating rejection of allogeneic tissues in vivo has not been fully explored. Using in vivo cytotoxicity assays, we evaluated the effector systems mediating the rejection of CFSE-labeled allogeneic splenocytes after an infection of C57BL/6 (B6) mice with lymphocytic choriomeningitis virus. Naive B6 mice predominantly used a NK cell-effector mechanism to reject allogeneic splenocytes because they rejected BALB/C (H2(d)) splenocytes but not CBA (H2(k)) splenocytes, and the rejection was prevented by immunodepletion of NK1.1(+) or Ly49D(+) NK cells. This rapid and efficient in vivo cytotoxicity assay recapitulated the specificity of NK cell-mediated rejection seen in longer duration in vivo assays. However, as early as 1 day after infection with lymphocytic choriomeningitis virus, a CD8 T cell-dependent mechanism participated in the rejection process and a broader range of tissue haplotypes (e.g., H2(k)) was susceptible. The CD8 T cell-mediated in vivo rejection process was vigorous at a time postinfection (day 3) when NK cell effector functions are peaking, indicating that the effector systems used in vivo differed from those observed with in vitro assays measuring the killing of allogeneic cells. This rapid generation of allospecific CTL activity during a viral infection preceded the peak of viral epitope-specific T cell responses, as detected by in vivo or in vitro cytotoxicity assays.     

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.     

A role for IFN-gamma in primary and secondary immunity generated by NK cell-sensitive tumor-expressing CD80 in vivo

We have investigated the primary and secondary immunity generated in vivo by a MHC class I-deficient tumor cell line that expressed CD80 (B7-1). CD80 expression enhanced primary NK cell-mediated tumor rejection in vivo and T cell immunity against secondary tumor challenge. CD80 expression enhanced primary NK cell-mediated tumor rejection, and both NK cell perforin and IFN-gamma activity were critical for the rejection of MHC class I-deficient RMA-S-CD80 tumor cells. This primary rejection process stimulated the subsequent development of specific CTL and Th1 responses against Ags expressed by the MHC class I-deficient RMA-S tumor cells. The development of effective secondary T cell immunity could be elicited by irradiated RMA-S-CD80 tumor cells and was dependent upon NK cells and IFN-gamma in the priming response. Our findings demonstrate a key role for IFN-gamma in innate and adaptive immunity triggered by CD80 expression on tumor cells.     

Effect of rabbit anti-asialo GM1 treatment in vivo or with anti-asialo GM1 plus complement in vitro on cytotoxic T cell activities

The susceptibility of cytotoxic effector lymphocytes and their induction to in vivo or in vitro treatment with rabbit anti-neutral glycolipid ganglio-N-tetraosylceramide (anti-ASGM1) antiserum was investigated. Intravenous injection of anti-ASGM1 antiserum eliminated measurable natural killer (NK) cell activity in spleen cells of mice infected for 5 days with Vaccinia virus, or for 8 days with lymphocytic choriomeningitis virus (LCMV) if injected 24 hr prior to testing. In addition, this treatment lowered measurable virus-specific cytotoxic T cell activity by 60 to 95%. Virus-specific cytotoxic T cell and NK cell activity generated during a primary infection in vivo was also sensitive to treatment in vitro with anti-ASGM1 antiserum (1/300 to 1/600 dilution) plus rabbit complement at a dilution of 1/15 (20 to 50% cell death, more than 30-fold decrease of cytotoxic activity); in vitro treatment with rabbit complement alone often enhanced NK and cytotoxic T cell activity slightly. In vivo treatment with anti-ASGM1 before primary immunization decreased generation of primary CTL only if high doses of anti-ASGM1 antiserum were injected twice. Antiviral T cells generated during secondary stimulation in vitro and alloreactive cytotoxic T cells from a mixed lymphocyte culture were resistant to treatment in vitro with anti-ASGM1 plus complement at the end of the culture period. Treatment in vitro of in vivo-primed responder spleen cells with anti-ASGM1 plus complement before their addition to a secondary restimulation culture resulted in complete inhibition of a secondary antiviral cytotoxic T cell response. In vivo treatment with anti-ASGM1 24 hr before their spleen cells were harvested and restimulated in vitro significantly reduced the virus-specific T cell activity of mice that had been immunized with virus several weeks previously. A cloned T cell line exclusively exerting NK-like activity was resistant, and two cloned virus-specific cytotoxic T cell lines were susceptible to treatment with anti-ASGM1 plus complement in vitro. These results caution the general use of rabbit anti-ASGM1 as a marker to distinguish NK from CTL cells; they indicate a possible relationship between NK and CTL cells and suggest that in vitro culture of lymphocytes may alter or select the cell surface expression or availability of the ASGM1 marker(s).     

Persistent virus infection despite chronic cytotoxic T-lymphocyte activation in gamma interferon-deficient mice infected with lymphocytic choriomeningitis virus

The role of gamma interferon (IFN-gamma) in the permanent control of infection with a noncytopathic virus was studied by comparing immune responses in wild-type and IFN-gamma-deficient (IFN-gamma -/-) mice infected with a slowly invasive strain of lymphocytic choriomeningitis virus (LCMV Armstrong). While wild-type mice rapidly cleared the infection, IFN-gamma -/- mice became chronically infected. Virus persistence in the latter mice did not reflect failure to generate cytotoxic T-lymphocyte (CTL) effectors, as an unimpaired primary CTL response was observed. Furthermore, while ex vivo CTL activity gradually declined in wild-type mice, long-standing cytolytic activity was demonstrated in IFN-gamma -/- mice. The prolonged effector phase in infected IFN-gamma -/- mice was associated with elevated numbers of CD8(+) T cells. Moreover, a higher proportion of these cells retained an activated phenotype and was actively cycling. However, despite the increased CD8(+) T-cell turnover, which might have resulted in depletion of the memory CTL precursor pool, no evidence for exhaustion was observed. In fact, at 3 months postinfection we detected higher numbers of LCMV-specific CTL precursors in IFN-gamma -/- mice than in wild-type mice. These findings indicate that in the absence of IFN-gamma, CTLs cannot clear the infection and are kept permanently activated by the continuous presence of live virus, resulting in a delicate new balance between viral load and immunity. This interpretation of our findings is supported by mathematical modeling describing the effect of eliminating IFN-gamma-mediated antiviral activity on the dynamics between virus replication and CTL activity.     

Veto activity of activated bone marrow does not require perforin and Fas ligand

Veto cells suppress generation of CD8(+) T cell immune responses in an antigen-specific manner, with specificity dictated by antigens on the veto cell surface. Activated bone marrow (ABM) veto cells belong to the NK cell type lineage and veto by clonally deleting antigen-specific precursor cytotoxic T cell lymphocyte (CTL). In vitro cytotoxicity of ABM depends largely on the perforin/granzyme and Fas/Fas ligand pathways. Utilizing perforin-deficient and functional Fas ligand-deficient gld mice as a source of ABM and functional Fas-deficient lpr mice as a source of precursor CTL, we demonstrate in this study that ABM cells utilize a perforin- and Fas-independent pathway to veto allogeneic cell-mediated cytotoxic responses. We also show that ABM cells mediate perforin- and Fas-independent veto activity even in an 8-h clonal deletion assay. We conclude that ABM veto activity does not require the two primary pathways of cell-mediated death.     

Anti-tumor necrosis factor antibodies suppress cell-mediated immunity in vivo.

Rabbit anti-murine TNF-alpha antibodies were administered in vivo to mice to evaluate the role of TNF-alpha in T cell-mediated immunity. Anti-TNF suppressed the in vivo development of contact sensitivity to the hapten TNP in a dose-dependent fashion. Similarly anti-TNF suppressed the in vivo priming for TNP-specific CTL. Control antibodies did not suppress cell-mediated immunity, whereas purified murine rTNF-alpha neutralized the antibody activity. Antibody therapy was effective during the afferent or priming limb of immunity, but could not inhibit the response if administered during the efferent limb. FACS for CD2, CD3, CD4, and CD8 T, B, and NK cell surface markers demonstrated no major change in the distribution of splenic lymphoid cell populations in animals pretreated with anti-TNF antibody. These results suggest that anti-TNF antibody may be interfering with soluble cytokines rather than with cell surface TNF causing depletion of cell populations. In vitro analyses also showed that anti-TNF has minimal inhibitory effects on secondary (secondary CTL) or strong primary (primary CTL, alpha CD3, MLR) responses, even though these in vitro cultures produce TNF mRNA as shown by polymerase chain reaction amplification. Although anti-TNF antibody did not affect the above responses, primary interactions are strongly inhibited in vivo. These findings suggest that TNF is important during afferent, priming events in immunity and that inhibition of TNF receptor-ligand interactions may alter immunity early in a response. Conversely such inhibition is ineffective later in a response, perhaps due to the ability of multiple other receptor-ligand pathways to bypass TNF.     

CD4-positive T lymphocytes are required for the generation of the primary but not the secondary CD8-positive cytolytic T lymphocyte response to herpes simplex virus in C57BL/6 mice.

To understand the cellular basis for recovery from HSV infection, it is critical to identify functional interactions between HSV-specific T lymphocyte subpopulations involved in the generation of the optimal response. To this end, the requirement for CD4+ (L3T4+) T lymphocytes in the development of the primary and secondary CD8+ (Lyt-2+) cytolytic T lymphocyte (CTL) response following HSV infection in C57BL/6 mice was investigated. It was found that chronic depletion of CD4+ cells in vivo by treatment with the mAb GK1.5, which resulted in greater than 95% depletion of peripheral CD4+ T lymphocytes in treated animals, caused a profound decrease in the levels of cytolytic activity obtained during the primary response in the draining popliteal lymph nodes of mice responding to infection in the hind footpads. However, treatment did not affect the levels of in vivo secondary CTL activity in the popliteal lymph nodes, nor the in vitro secondary response in the spleen. The decreased CTL activity observed during the primary response was not due to an inability to prime HSV-specific CTL precursors (CTLp), as full cytolytic activity was obtained following culture of lymphocytes in the presence of exogenous IL-2 and antigen, and the response could be reconstituted by treatment with recombinant IL-2 in vivo. Analysis of the secondary CTL response in the spleen indicated that CD4+ cells were not required for either the generation or maintenance of this aspect of the response. However, blockade of IL-2 utilization by CTL using anti-IL-2R antibodies indicated that this lymphokine was absolutely essential for secondary CTL expansion in vitro. Finally, mice that had been infected 12 months previously exhibited a decreased ability to generate secondary HSV-specific CTL in vitro following CD4-depletion in vivo. Taken together, these results suggest two distinct stages of CTL development during the response: an early primary stage dependent upon the presence of CD4+ cells, and a later, CD4-independent stage operative during the secondary response, which decays with time postinfection.     

Alveolar macrophages regulate the induction of primary cytotoxic T-lymphocyte responses during influenza virus infection.

Virus-specific cytotoxic T lymphocytes (CTL) are thought to be responsible for the eradication of respiratory influenza virus infections by direct cytolysis of virus-infected epithelial cells. In this study, we provide evidence for a role for alveolar macrophages (AM) in the regulation of pulmonary virus-specific CTL responses. Prior to infection with influenza virus, AM were selectively eliminated in vivo with a liposome-mediated depletion technique, and virus-specific CTL activities of lung and mediastinal lymph node (MLN) cells were assayed ex vivo and compared with those for normal mice. AM depletion resulted in increased primary CTL responses and changed the kinetics of the CTL response. Flow cytometric analysis of lung and MLN cells showed that the percentage of CD8+ cells was not altered after AM depletion and that lung cells from AM-depleted mice had an increased capacity to lyse virus-infected cells. Upon restimulation in vitro, virus-specific CTL activity in lung cells of normal mice was similar to that in lung cells of AM-depleted mice. Furthermore, elimination of AM resulted in increased virus titers in the lung, but virus clearance as a function of time was not affected. Our results show that AM regulate virus-specific CTL responses during respiratory influenza virus infection by removing viral particles, by downregulating the priming and activity of CTL in MLN cells, and by inhibiting the expansion of virus-specific CTL in the lung.     

Enhancement of primary and secondary cellular immune responses against human immunodeficiency virus type 1 gag by using DNA expression vectors that target Gag antigen to the secretory pathway

In this study, we have investigated the influence of antigen targeting after DNA vaccination upon the induction of cellular immune responses against human immunodeficiency virus type 1 (HIV-1) Gag. In addition to the standard version of HIV-1 Gag, we constructed Gag expression vectors that encode a secreted (Sc-Gag) and a cytoplasmic (Cy-Gag) Gag molecule. Although all three HIV-1 Gag expression vectors induced detectable humoral and cellular immune responses, after intramuscular injection the DNA vector encoding the Sc-Gag generated the highest primary cytotoxic T-lymphocyte (CTL) and T-helper responses. Mice immunized with one of the HIV-1 Gag DNA vectors (but not with the control vector pcDNA3. 1) developed a protective immune response against infection with recombinant vaccinia virus expressing HIV-1 Gag, and this response persisted for 125 days. The magnitude of the protection correlated with the levels of Gag-specific ex vivo CTL activity and the number of CD8(+) T cells producing gamma interferon. The DNA vector encoding the Sc-Gag induced higher levels of protection and greater secondary CTL responses than did the DNA vector encoding Cy-Gag.     

Cell-mediated cytotoxicity: comparison of primary and secondary immune reactions after parainfluenza type 1 virus inoculation.

H-2 antigen compatibility of effector spleen cells and target cells was necessary for cell-mediated cytotoxicity (CMC) after both 1 and 2 intraperitoneal inoculation of 6/94 virus into mice. T cells were responsible for both primary and secondary CMS reactions. Neither reaction was influenced by the presence of virus-specific antibody added to the test system. Secondary CMC peaked earlier than the primary response and declined more rapidly. In vitro stimulation of primed spleen cells could also be used to detect the secondary CMC response.     

Vaccinia virus interleukin-18-binding protein promotes virulence by reducing gamma interferon production and natural killer and T-cell activity

Interleukin-18 (IL-18) is a proinflammatory cytokine that promotes natural killer (NK) and T-cell activation. Several poxviruses, including vaccinia virus (VV), encode a soluble IL-18-binding protein (IL-18bp). The role of the VV IL-18bp (gene C12L) in vivo was studied with wild-type (vC12L), deletion mutant (vDeltaC12L), and revertant (vC12L-rev) viruses in a murine intranasal model of infection. The data show that vDeltaC12L was markedly attenuated, characterized by a mild weight loss and reduced virus titers in lungs, brain, and spleen. Three days after infection, NK cytotoxic activity was augmented in the lung, spleen, and mediastinal lymph nodes (MLNs) of vDeltaC12L-infected mice compared to controls. Seven days after infection, vDeltaC12L-infected mice displayed heightened VV-specific cytotoxic T-lymphocyte (CTL) responses in the lungs, spleen, and MLNs. Gamma interferon (IFN-gamma) levels were also dramatically elevated in lavage fluids and cells from lungs of mice infected with vDeltaC12L. Finally, we demonstrate that IL-18 is produced in vitro and in vivo after VV infection. Taken together, these data demonstrate a role for the vIL-18bp in counteracting IL-18 in both the innate and the specific immune response to VV infection and indicate that the ability of IL-18 to promote vigorous T-cell responses (cytotoxic activity and IFN-gamma production) is a critical factor in the accelerated clearance of the vDeltaC12L mutant.     

Augmentation of cellular immune response by Ipomoea obscura and Ipobscurine alkaloid attenuates tumor growth in mice

The immune status of the host plays a crucial role in controling the process of carcinogenesis. General or selective activation of various immunocompetent cells and their secretory function to maintain a healthy immune status may help in cancer prophylaxis, as well as therapy. The present study focused on the effect of Ipomoea obscura and Ipobscurine on cell-mediated immune response. In this study we evaluated the effect of I.?obscura and an indole alkaloid fraction from I. obscura on effector mechanisms of cell-mediated immune response by analyzing cytotoxic T lymphocyte (CTL) activity, natural killer (NK) cell activity, antibody-dependent cell-mediated cytotoxicity (ADCC), and antibody-dependent complement-mediated cytotoxicity (ACC). The effect of I. obscura and Ipobscurine on interleukin-2 (IL-2) and interferon-γ (IFN-γ) levels was also analyzed. In the in vitro and in vivo systems, I. obscura and Ipobscurine treatment augmented cell-mediated immune response by enhancing the killing activity of CTL and NK cells from splenocytes in normal as well as tumor-bearing mice. ADCC and ACC were also enhanced significantly in both normal and tumor-bearing animals after drug administration, compared with untreated control. Administration of I. obscura and Ipobscurine significantly enhanced the production of IL-2 and IFN-γ in normal as well as tumor-bearing animals. This study reveals that both I. obscura and Ipobscurine have the potential to augment immune response through the enhanced secretion of IL-2 and IFN-γ by T cells and thereby inhibit tumor growth and as an alternative medicine for cancer treatment.     

Vaccine-induced memory CD8+ T cells cannot prevent central nervous system virus reactivation

Noncytopathic viruses use multiple strategies to evade immune detection, challenging a role for vaccine induced CTL in preventing microbial persistence. Recrudescence of neurotropic coronavirus due to loss of T cell-mediated immune control provided an experimental model to test T cell vaccination efficacy in the absence of Ab. Challenge virus was rapidly controlled in vaccinated Ab-deficient mice coincident with accelerated recruitment of memory CD8+ T cells and enhanced effector function compared with primary CD8+ T cell responses. In contrast to primary effectors, reactivated memory cells persisted in the CNS at higher frequencies and retained ex vivo cytolytic activity. Nevertheless, despite earlier and prolonged T cell-mediated control in the CNS of vaccinated mice, virus ultimately reactivated. Apparent loss of memory CD8+ effector function in vivo was supported by a prominent decline in MHC expression on CNS resident target cells, presumably reflecting diminished IFN-gamma. Severely reduced MHC expression on glial cells at the time of recrudescence suggested that memory T cells, although fully armed to exert antiviral activity upon Ag recognition in vitro, are not responsive in an environment presenting few if any target MHC molecules. Paradoxically, effective clearance of viral Ag thus affords persisting virus a window of opportunity to escape from immune surveillance. These studies demonstrate that vaccine-induced T cell memory alone is unable to control persisting virus in a tissue with strict IFN-dependent MHC regulation, as evident in immune privileged sites.     

T lymphocyte immunity to reovirus: cellular requirements for generation and role in clearance of primary infections

The cellular requirements for the development of an immune response to reovirus type 1 and the role of such a response in the clearance of a primary infection with that virus were explored. An Ia-bearing antigen-presenting cell requirement is demonstrated for the in vitro generation of secondary anti-reovirus cytolytic T lymphocytes (CTL). It is then shown that mice whose spleens are depleted of Ia-bearing adherent cells by exposure in vivo to ultraviolet (UV) radiation exhibit depressed priming for reovirus-specific T lymphocyte function-CTL generation, delayed-type hypersensitivity reactivity, and T cell proliferative responsiveness. These UV-irradiated mice clear primary systemic reovirus infections as readily as normal mice. Further, athymic 'nude' mice show no defect in their ability to clear a reovirus infection. The implications of these findings for our understanding of the role of virus-specific T cell function in the clearance of systemic viral infections are discussed.     

Interleukin-4 causes delayed virus clearance in influenza virus-infected mice.

Two different subsets of T cells, Th1 and Th2 cells, have been demonstrated to secrete different profiles of cytokines and to influence various infections in different ways. Whereas cytokines secreted by Th1 cells, particularly gamma interferon, promote the generation of cell-mediated immunity, Th2 cells and their cytokines (interleukin-4 [IL-4], IL-5, IL-10, and IL-13) have been shown to function in recovery from parasitic infections and in antibody responses. In this study, we analyzed the effects of the dominant Th2 cytokine, IL-4, on immunity to virus infection. We assessed the effects of IL-4 on both secondary immune responses by an adoptive transfer assay and primary immune responses by in vivo treatment of influenza virus-infected mice with IL-4. The results demonstrated that IL-4 can function to inhibit antiviral immunity at both stages. We found that IL-4 treatment of sensitized cells during secondary stimulation in vitro had little effect on their ability to lyse virus-infected target cells in a 51Cr release assay. Nevertheless, the clearance of influenza A/PR/8/34 (H1N1) virus from the lungs of infected BALB/c mice was significantly delayed after the transfer of virus-specific T cells secondarily stimulated in the presence of IL-4 in comparison to virus clearance in recipients of cells stimulated in the absence of IL-4. In contrast to the adoptive transfer results, the treatment of PR8 virus-infected mice with IL-4 during primary infection greatly suppressed the generation of cytotoxic T-cell precursors, as assessed by secondary stimulation in vitro. In addition, culture supernatants of secondarily stimulated spleen cells from IL-4-treated mice contained significantly less gamma interferon and more IL-4 than did spleen cells from controls. More importantly, the treatment of mice with IL-4 resulted in an extremely significant delay in virus clearance. Thus, IL-4 can inhibit both primary and secondary antiviral immune responses.     

Cell-mediated immunity to vesicular stomatitis virus infections in mice.

The T cell-mediated immune responses of mice against vesicular stomatitis virus (VSV) were assessed by measuring direct primary foot pad swelling after local VSV infection and cytotoxic activity in spleens. The cytolytic activity was mediated by T cells since it was anti-theta + complement sensitive, was restricted by the K and D region but not the I region of H-2 and rapidly increased after 4 days but decreased 8 days after systemic or local infection. Cytolytic activity was virus-specific as reciprocally tested with VSV and vaccina virus immune T cells. Measurable activity on day 7 depended on infectious virus dose, virus virulence, and non-H-2 genetic background of the host. More than half of the cytolytic activity wasblocked specifically by either immune anti-H2 or rabbit anti-VSV antisera. Analysis of the kinetics of appearance of antigenic changes using metabolic inhibitors, revealed that the changes that rendered target cells susceptible to lysis after infection, occurred within the first hour after infection.     

A new theory of cytotoxic T-lymphocyte memory: implications for HIV treatment

We use simple mathematical models to examine the dynamics of primary and secondary cytotoxic T-lymphocyte (CTL) responses to viral infections. In particular, we are interested in conditions required to resolve the infection and to protect the host upon secondary challenge. While protection against reinfection is only effective in a restricted set of circumstances, we find that resolution of the primary infection requires persistence of CTL precursors (GTLp), as well as a fast rate of activation of the CTLp. Since these are commonly the defining characteristics of CTL memory, we propose that CTL memory may have evolved in order to clear the virus during primary challenge. We show experimental data from lymphocytic choriomeningitis virus infection in mice, supporting our theory on CTL memory. We adapt our models to HIV and find that immune impairment during the primary phase of the infection may result in the failure to establish CTL memory which in turn leads to viral persistence. Based on our models we suggest conceptual treatment regimes which ensure establishment of CTL memory. This would allow the immune response to control HIV in the long term in the absence of continued therapy.