Cutting Edge: The direct action of type I IFN on CD4 T cells is critical for sustaining clonal expansion in response to a viral but not a bacterial infection

The action of type I IFN (IFN-I) on APCs is well studied, but their direct effect on CD4 T cells is unclear. To address this, we transferred IFN-I receptor-deficient (IFN-IR(0)) and -sufficient (wild-type, WT) TCR-transgenic CD4 T cells into WT mice and analyzed their response to immunization. In response to lymphocytic choriomeningitis virus immunization, WT CD4 T cells expanded approximately 100-fold, whereas IFN-IR(0) CD4 T cells expanded <10-fold. However, both WT and IFN-IR(0) CD4 T cells expanded approximately 10-fold after Listeria monocytogenes immunization. Poor expansion of IFN-IR(0) CD4 T cells after lymphocytic choriomeningitis virus immunization was not due to a defect in proliferation or initial activation but to poor survival of the daughter cells. Thus, direct IFN-I signals can play either a critical or minimal role in CD4 T cell clonal expansion depending on the specific pathogen.     

Protection against vaccinia virus challenge by CD8 memory T cells resolved by molecular mimicry

Live vaccinia virus (VV) vaccination has been highly successful in eradicating smallpox. However, the mechanisms of immunity involved in mediating this protective effect are still poorly understood, and the roles of CD8 T-cell responses in primary and secondary VV infections are not clearly identified. By applying the concept of molecular mimicry to identify potential CD8 T-cell epitopes that stimulate cross-reactive T cells specific to lymphocytic choriomeningitis virus (LCMV) and VV, we identified after screening only 115 peptides two VV-specific immunogenic epitopes that mediated protective immunity against VV. An immunodominant epitope, VV-e7r130, did not generate cross-reactive T-cell responses to LCMV, and a subdominant epitope, VV-a11r198, did generate cross-reactive responses to LCMV. Infection with VV induced strong epitope-specific responses which were stable into long-term memory and peaked at the time virus was cleared, consistent with CD8 T cells assisting in the control of VV. Two different approaches, direct adoptive transfer of VV-e7r-specific CD8 T cells and prior immunization with a VV-e7r-expressing ubiquitinated minigene, demonstrated that memory CD8 T cells alone could play a significant role in protective immunity against VV. These studies suggest that exploiting cross-reactive responses between viruses may be a useful tool to complement existing technology in predicting immunogenic epitopes to large viruses, such as VV, leading to a better understanding of the role CD8 T cells play during these viral infections.     

Timing and magnitude of type I interferon responses by distinct sensors impact CD8 T cell exhaustion and chronic viral infection

Type I interferon (IFN-I) promotes antiviral CD8(+)T cell responses, but the contribution of different IFN-I sources and signaling pathways are ill defined. While plasmacytoid dendritic cells (pDCs) produce IFN-I upon TLR stimulation, IFN-I is induced in most cells by helicases like MDA5. Using acute and chronic lymphocytic choriomeningitis virus (LCMV) infection models, we determined that pDCs transiently produce IFN-I that minimally impacts CD8(+)T cell responses and viral persistence. Rather, MDA5 is the key sensor that induces IFN-I required for CD8(+)T cell responses. In the absence of MDA5, CD8(+)T cell responses to acute infection rely on CD4(+)T cell help, and loss of both CD4(+)T cells and MDA5 results in CD8(+)T cell exhaustion and persistent infection. Chronic LCMV infection rapidly attenuates IFN-I responses, but early administration of exogenous IFN-I rescues CD8(+)T cells, promoting viral clearance. Thus, effective antiviral CD8(+)T cell responses depend on the timing and magnitude of IFN-I production.     

IL-12 and type-I IFN synergize for IFN-gamma production by CD4 T cells, whereas neither are required for IFN-gamma production by CD8 T cells after Listeria monocytogenes infection

Differentiation of Ag-specific T cells into IFN-gamma producers is essential for protective immunity to intracellular pathogens. In addition to stimulation through the TCR and costimulatory molecules, IFN-gamma production is thought to require other inflammatory cytokines. Two such inflammatory cytokines are IL-12 and type I IFN (IFN-I); both can play a role in priming naive T cells to produce IFN-gamma in vitro. However, their role in priming Ag-specific T cells for IFN-gamma production during experimental infection in vivo is less clear. In this study, we examine the requirements for IL-12 and IFN-I, either individually or in combination, for priming Ag-specific T cell IFN-gamma production after Listeria monocytogenes (Lm) infection. Surprisingly, neither individual nor combined defects in IL-12 or IFN-I signaling altered IFN-gamma production by Ag-specific CD8 T cells after Lm infection. In contrast, individual defects in either IL-12 or IFN-I signaling conferred partial ( approximately 50%) reductions, whereas combined deficiency in both IL-12 and IFN-I signaling conferred more dramatic (75-95%) reductions in IFN-gamma production by Ag-specific CD4 T cells. The additive effects of IL-12 and IFN-I signaling on IFN-gamma production by CD4 T cells were further demonstrated by adoptive transfer of transgenic IFN-IR(+/+) and IFN-IR(-/-) CD4 T cells into normal and IL-12-deficient mice, and infection with rLm. These results demonstrate an important dichotomy between the signals required for priming IFN-gamma production by CD4 and CD8 T cells in response to bacterial infection.     

Role of CD40 ligand and CD28 in induction and maintenance of antiviral CD8+ effector T cell responses

The primary aim of this report was to evaluate the immune responses of CD40 ligand-deficient (CD40L-/-) mice infected with two viruses known to differ markedly in their capacity to replicate in the host. Lymphocytic choriomeningitis virus (LCMV) is a natural mouse pathogen that replicates widely and extensively, whereas vesicular stomatitis virus (VSV) spreads poorly. We found that the primary response of CD40L-/- mice toward VSV is significantly impaired; proliferation of both CD4+ and CD8+ cells is reduced 2- to 3-fold, few CD8+ cells acquire an activated phenotype, and little functional activity is induced. Very similar results were obtained in VSV-infected, CD28-deficient mice. In contrast, neither CD40L nor CD28 was required for induction of a primary CD8+ response toward LCMV. Surprisingly, lack of CD4+ T cells had no impact on the primary immune response toward any of the viruses, even though the CD40 ligand dependence demonstrated for VSV would be expected to be associated with CD4 dependence. Upon coinfection of VSV-infected mice with LCMV, the requirement for CD40 ligand (but not CD28) could be partially bypassed, as evidenced by a 3-fold increase in the frequency of VSV-specific CD8+ T cells on day 6 postinfection. Finally, despite the fact that the primary LCMV-specific CD8+ response is virtually unimpaired in CD40L-/- mice, their capacity to maintain CD8+ effector activity and to permanently control the infection is significantly reduced. Thus, our results demonstrate that the importance of CD40/CD40L interaction for activation of CD8+ T cells varies between viruses and over time.     

Deficient anti-listerial immunity in the absence of perforin can be restored by increasing memory CD8+ T cell numbers

Compared with wild-type (WT) mice, Listeria monocytogenes (LM)-vaccinated perforin-deficient (PKO) mice have elevated levels of CD8(+) T cell memory, but exhibit reduced levels of protection against virulent LM. In this study, Ag-specific CD8(+) T cells from LM-vaccinated WT and PKO mice were used in adoptive transfer assays to determine the contribution of perforin-dependent cytolysis in protective immunity to LM. Perforin deficiency resulted in an approximately 5-fold reduction in the per-cell protective capacity of Ag-specific memory CD8(+) T cells that was not caused by differences in memory cell quality as measured by CD62L/CD27 expression, TCR repertoire use, functional avidity, differences in expansion of Ag-specific cells upon infection, or maintenance of memory levels over time. However, perforin-deficient CD8(+) T cells exhibited reduced in vivo cytotoxic function compared to WT CD8(+) T cells. Consistent with the existence of perforin-independent effector pathways, double-vaccinated PKO mice were as resistant to challenge with LM as single-vaccinated WT mice. Thus, increasing the number of memory CD8(+) T cells can overcome diminished per-cell protective immunity in the absence of perforin.     

Inhibitory effects of cytomegalovirus proteins US2 and US11 point to contributions from direct priming and cross-priming in induction of vaccinia virus-specific CD8(+) T cells

The extent to which naive CD8(+) CTLs (T(CD8)(+)) are primed by APCs presenting endogenous Ags (direct priming) or Ags acquired from other infected cells (cross-priming) is a critical topic in basic and applied immunology. To examine the contribution of direct priming in the induction of VV-specific T(CD8)(+), we generated recombinant vaccinia viruses that express human CMV proteins (US2 and US11) that induce the destruction of newly synthesized MHC class I molecules. Expression of US2 or US11 was associated with a 24-63% decrease in numbers of primary or secondary VV-specific T(CD8)(+) responding to i.p. infection. Using HPLC-isolated peptides from VV-infected cells, we show that US2 and US11 selectively inhibit T(CD8)(+) responses to a subset of immunogenic VV determinants. Moreover, VV-US2 and lysates from VV-infected histoincompatible cells elicit T(CD8)(+) specific for a similar subset of VV determinants. These findings indicate that US2 and US11 can function in vivo to interfere with the activation of virus-specific T(CD8)(+). Furthermore, they suggest that 1) both cross-priming and direct priming contribute significantly to the generation of VV-specific T(CD8)(+), 2) the sets of immunogenic vaccinia virus determinants generated by cross-priming and direct priming are not completely overlapping, and 3) cross-priming overrides the effects of cis-acting viral interference with the class I Ag presentation pathway.     

CD8 T cells specific for lymphocytic choriomeningitis virus require type I IFN receptor for clonal expansion

The role of type I IFN signaling in CD8 T cells was analyzed in an adoptive transfer model using P14 TCR transgenic CD8 T cells specific for lymphocytic choriomeningitis virus (LCMV) but deficient in type I IFNR. In the present study, we demonstrate severe impairment in the capacity of P14 T cells lacking type I IFNR to expand in normal type I IFNR wild-type C57BL/6 hosts after LCMV infection. In contrast, following infection of recipient mice with recombinant vaccinia virus expressing LCMV glycoprotein, P14 T cell expansion was considerably less dependent on type I IFNR expression. Lack of type I IFNR expression by P14 T cells did not affect cell division after LCMV infection but interfered with clonal expansion. Thus, direct type I IFN signaling is essential for CD8 T cell survival in certain viral infections.     

Dynamics of memory T cell proliferation under conditions of heterologous immunity and bystander stimulation

By examining adoptively transferred CSFE-labeled lymphocytic choriomeningitis virus (LCMV)-immune donor T cells in Thy-1 congenic hosts inoculated with viruses or with the cytokine inducer poly(I:C), strikingly different responses of bona fide memory T cells were found in response to different stimuli. Poly(I:C) (cytokine) stimulation caused a limited synchronized division of memory CD8 T cells specific to each of five LCMV epitopes, with no increase and sometimes a loss in number, and no change in their epitope hierarchy. Homologous LCMV infection caused more than seven divisions of T cells specific for each epitope, with dramatic increases in number and minor changes in hierarchy. Infections with the heterologous viruses Pichinde and vaccinia (VV) caused more than seven divisions and increases in number of T cells specific to some putatively cross-reactive but not other epitopes and resulted in substantial changes in the hierarchy of the LCMV-specific T cells. Hence, there can be memory T cell division without proliferation (i.e., increase in cell number) in the absence of Ag and division with proliferation in the presence of Ag from homologous or heterologous viruses. Heterologous protective immunity between viruses is not necessarily reciprocal, given that LCMV protects against VV but VV does not protect against LCMV. VV elicited proliferation of LCMV-induced CD8 and CD4 T cells, whereas LCMV did not elicit proliferation of VV-induced T cells. Thus, depending on the pathogen and the sequence of infection, a heterologous agent may selectively stimulate the memory pool in patterns consistent with heterologous immunity.     

Quantifying antigen-specific CD4 T cells during a viral infection: CD4 T cell responses are larger than we think

The number of virus-specific CD8 T cells increases substantially during an acute infection. Up to 90% of CD8 T cells are virus specific following lymphocytic choriomeningitis virus (LCMV) infection. In contrast, studies identifying virus-specific CD4 T cell epitopes have indicated that CD4 T cells often recognize a broader array of Ags than CD8 T cells, consequently making it difficult to accurately quantify the total magnitude of pathogen-specific CD4 T cell responses. In this study, we show that CD4 T cells become CD11a(hi)CD49d(+) after LCMV infection and retain this expression pattern into memory. During the effector phase, all the LCMV-specific IFN-γ(+) CD4 T cells display a CD11a(hi)CD49d(+) cell surface expression phenotype. In addition, only memory CD11a(hi)CD49d(+) CD4 T cells make IFN-γ after stimulation. Furthermore, upon secondary LCMV challenge, only CD11a(hi)CD49d(+) memory CD4 T cells from LCMV-immune mice undergo proliferative expansion, demonstrating that CD11a(hi)CD49d(+) CD4 T cells are truly Ag specific. Using the combination of CD11a and CD49d, we demonstrate that up to 50% of the CD4 T cells are virus specific during the peak of the LCMV response. Our results indicate that the magnitude of the virus-specific CD4 T cell response is much greater than previously recognized.     

4-1BB ligand, a member of the TNF family, is important for the generation of antiviral CD8 T cell responses.

4-1BB (CD137) is a costimulatory molecule expressed on activated T cells and interacts with 4-1BB ligand (4-1BBL) on APCs. To investigate the role of 4-1BB costimulation for the development of primary immune responses, 4-1BBL-deficient (4-1BBL-/-) mice were infected with lymphocytic choriomeningitis virus (LCMV). 4-1BBL-/- mice were able to generate CTL and eliminate acute LCMV infection with normal kinetics, but CD8 T cell expansion was 2- to 3-fold lower than in wild-type (+/+) mice. In the same mice, virus-specific CD4 Th and B cell responses were minimally affected, indicating that 4-1BB costimulation preferentially affects CD8 T cell responses. This result contrasts with our earlier work with CD40L-deficient (CD40L-/-) mice, in which the CD8 T cell response was unaffected and the CD4 T cell response was markedly impaired. When both 4-1BBL- and B7-dependent signals were absent, CD8 T cell expansion was further reduced, resulting in lower CTL activity and impairing their ability to clear LCMV. Altogether, these results indicate that T cells have distinct costimulatory requirements: optimal CD8 responses require 4-1BBL-dependent interactions, whereas CD4 responses are minimally affected by 4-1BB costimulation, but require CD40-CD40L and B7-dependent interactions.     

Imatinib mesylate inhibits antigen-specific memory CD8 T cell responses in vivo

Imatinib mesylate (IM) is effective at inducing complete cytogenetic remission in patients with chronic myelogenous leukemia. Because its influence on CD8 T cell responsiveness in vivo is unknown, we investigated the effects of IM by analyzing the response of OT-1 CD8 T cells to Listeria monocytogenes (LM) that express the cognate epitope OVA(257-264) (LM-OVA). In vitro, IM had no effect on Ag-specific expansion, cell division, cell cycle progression, or IFN-gamma expression in naive or memory OT-1 T cells. However, IM induced apoptosis of naive and memory OT-1 T cells at doses of >5 microM. At 15 microM IM, OT-1 T cells did not survive in in vitro cultures. The primary response of OT-1 T cells in vivo to LM-OVA infection was unaltered. In contrast, continuous IM treatment resulted in a diminished memory OT-1 response. The expression of IL-7Ralpha, a receptor required for memory cell survival, was lower (on OT-1 cells) in animals receiving IM. These results indicate that IM treatment affects the ability of the CD8 memory pool to respond to Ag and has the potential to increase susceptibility to infection.     

Direct visualization of cross-reactive effector and memory allo-specific CD8 T cells generated in response to viral infections

CD8 T cell cross-reactivity between heterologous viruses has been shown to provide protective immunity, induce immunopathology, influence the immunodominance of epitope-specific T cell responses, and shape the overall memory population. Virus infections also induce cross-reactive allo-specific CTL responses. In this study, we quantified the allo-specific CD8 T cells elicited by infection of C57BL/6 (B6) mice with lymphocytic choriomeningitis virus (LCMV). Cross-reactive LCMV-specific CD8 T cells were directly visualized using LCMV peptide-charged MHC tetramers to costain T cells that were stimulated to produce intracellular IFN-gamma in response to allogeneic target cells. The cross-reactivity between T cells specific for LCMV and allogeneic Ags was broad-based, in that it involved multiple LCMV-derived peptides, but there were distinctive patterns of reactivity against allogeneic cells with different haplotypes. Experiments indicated that this cross-reactivity was not due to the expression of two TCR per cell, and that the patterns of allo-reactivity changed during sequential infection with heterologous viruses. The allo-specific CD8 T cells generated by LCMV infection were maintained at relatively high frequencies in the memory pool, indicating that memory allo-specific CD8 T cell populations can arise as a consequence of viral infections. Mice previously infected with LCMV and harboring allo-specific memory T cells were refractory to the induction of tolerance to allogeneic skin grafts.     

An MHC class Ib-restricted CD8+ T cell response to lymphocytic choriomeningitis virus

Conventional MHC class Ia-restricted CD8(+) T cells play a dominant role in the host response to virus infections, but recent studies indicate that T cells with specificity for nonclassical MHC class Ib molecules may also participate in host defense. To investigate the potential role of class Ib molecules in anti-viral immune responses, K(b-/-)D(b-/-)CIITA(-/-) mice lacking expression of MHC class Ia and class II molecules were infected with lymphocytic choriomeningitis virus (LCMV). These animals have a large class Ib-selected CD8(+) T cell population and they were observed to mediate partial (but incomplete) virus clearance during acute LCMV infection as compared with K(b-/-)D(b-/-)β(2)-microglobulin(-/-) mice that lack expression of both MHC class Ia and class Ib molecules. Infection was associated with expansion of splenic CD8(+) T cells and induction of granzyme B and IFN-γ effector molecules in CD8(+) T cells. Partial virus clearance was dependent on CD8(+) cells. In vitro T cell restimulation assays demonstrated induction of a population of β(2)-microglobulin-dependent, MHC class Ib-restricted CD8(+) T cells with specificity for viral Ags and yet to be defined nonclassical MHC molecules. MHC class Ib-restricted CD8(+) T cell responses were also observed after infection of K(b-/-)D(b-/-)mice despite the low number of CD8(+) T cells in these animals. Long-term infection studies demonstrated chronic infection and gradual depletion of CD8(+) T cells in K(b-/-)D(b-/-)CIITA(-/-) mice, demonstrating that class Ia molecules are required for viral clearance. These findings demonstrate that class Ib-restricted CD8(+) T cells have the potential to participate in the host immune response to LCMV.     

Origin and fate of lymphocytic choriomeningitis virus-specific CD8+ T cells coexpressing the inhibitory NK cell receptor Ly49G2

CD8+ T cells that coexpress the inhibitory NK cell receptor, Ly49G2 (G2), are present in immunologically naive C57BL/6 mice but display Ags found on memory T cells. To assess how G2+CD8+ cells relate to bona fide memory cells, we examined the origin and fate of lymphocytic choriomeningitis virus (LCMV)-induced G2+CD8+ cells. During early (day 4) acute LCMV infection, both G2+ and G2-CD8+ T cell subsets underwent an attrition in number and displayed an activation (CD69(high)1B11(high)CD62L(low)) phenotype. By day 8, both subsets synthesized IFN-gamma in response to immunodominant LCMV peptides, though the expansion of G2+ cells was less than that of G2- cells. Adoptive transfer experiments with purified G2- or G2+CD8+ cells from naive mice indicated that the LCMV-specific G2+ subset was derived from a pre-existing G2+ population and not generated from G2- cells responding to LCMV infection. Their participation in the LCMV-specific T cell response increased with age, reflecting an increase in the size of the pre-existing G2+ pool. Following establishment of stable LCMV memory, the proportion of CD8+ cells coexpressing G2 was reduced in comparison to naive controls, presumably due to displacement by G2- LCMV-specific memory cells. LCMV-specific G2+ cells were present in the memory pool, but at low frequencies, and they did not exhibit the typical phenotypic changes of reactivation during secondary challenge. We suggest that G2+CD8+ cells represent a cell lineage distinct from bona fide memory T cells, but that they can participate in an acute virus-specific T cell response.     

Antiviral CD4 and CD8 T-cell memory: differences in the size of the response and activation requirements

Following acute lymphocytic choriomeningitis virus (LCMV) infection, there is a potent antiviral CD8 T-cell response that eliminates the infection. This initial CD8 T-cell response is followed by a period of memory during which elevated numbers of virus-specific CD8 T cells remain in the mouse. CD4 T cells are also activated after LCMV infection, but relatively less is known about the magnitude and duration of the CD4 response. In this study, we used intracellular staining for interferon-gamma to measure both CD4 and CD8 responses in the same mice at the single cell level. After LCMV infection, there was an increase in the number of activated CD4 T cells and an associated increase in the number of virus-specific CD4 T cells. At the peak of this expansion phase, the frequency of virus-specific CD4 T cells was 1 in 20 (0.5-1.0 x 10(6) per spleen). Like the CD8 response, long-term CD4 memory could be found up to a year after the infection with frequencies of approximately 1 in 260 (0.5-1.5 x 10(5) per spleen). However, the magnitude of virus-specific CD8 T cells was greater than virus-specific CD4 T cells during all phases of the immune response (expansion, death, and memory). At day 8, there were 20- to 35-fold more virus-specific CD8 T cells than CD4 T cells. This initial difference in cell number lasted into the memory phase as there remained a ten- to 20-fold difference in the CD8 and CD4 responses. These results highlight the importance of the expansion phase in determining the size of the memory T-cell pool. In addition to the difference in the magnitude, the activation requirements of CD8 and CD4 T-cell responses were different: CD8 T responses were not affected by blockade of CD40-CD40 ligand interaction whereas CD4 responses were reduced 90%. So while there is long-term memory in both the CD8 and CD4 compartments, the rules regulating the activation of CD8 and CD4 T cells and the overall magnitude of the responses are different.     

Type I IFNs control antigen retention and survival of CD8α(+) dendritic cells after uptake of tumor apoptotic cells leading to cross-priming

Cross-presentation is a crucial mechanism for generating CD8 T cell responses against exogenous Ags, such as dead cell-derived Ag, and is mainly fulfilled by CD8α(+) dendritic cells (DC). Apoptotic cell death occurring in steady-state conditions is largely tolerogenic, thus hampering the onset of effector CD8 T cell responses. Type I IFNs (IFN-I) have been shown to promote cross-priming of CD8 T cells against soluble or viral Ags, partly through stimulation of DC. By using UV-irradiated OVA-expressing mouse EG7 thymoma cells, we show that IFN-I promote intracellular Ag persistence in CD8α(+) DC that have engulfed apoptotic EG7 cells, regulating intracellular pH, thus enhancing cross-presentation of apoptotic EG7-derived OVA Ag by CD8α(+) DC. Notably, IFN-I also sustain the survival of Ag-bearing CD8α(+) DC by selective upmodulation of antiapoptotic genes and stimulate the activation of cross-presenting DC. The ensemble of these effects results in the induction of CD8 T cell effector response in vitro and in vivo. Overall, our data indicate that IFN-I cross-prime CD8 T cells against apoptotic cell-derived Ag both by licensing DC and by enhancing cross-presentation.     

Recombinant vaccinia virus-induced T-cell immunity: quantitation of the response to the virus vector and the foreign epitope

Recombinant vaccinia viruses (rVV) have been extensively used as vaccines, but there is little information about the total magnitude of the VV-specific T-cell response and how this compares to the immune response to the foreign gene(s) expressed by the rVV. To address this issue, we quantitated the T-cell responses to both the viral vector and the insert following the infection of mice with VV expressing a cytotoxic T lymphocyte (CTL) epitope (NP118-126) from lymphocytic choriomeningitis virus (LCMV). The LCMV epitope-specific response was quantitated by intracellular cytokine staining after stimulation with the specific peptide. To analyze the total VV-specific response, we developed a simple intracellular cytokine staining assay using VV-infected major histocompatibility complex class I and II matched cells as stimulators. Using this approach, we made the following determinations. (i) VV-NP118 induced potent and long-lasting CD8 and CD4 T-cell responses to the vector; at the peak of the response (approximately 1 week), there were approximately 10(7) VV-specific CD8 T cells (25% of the CD8 T cells) and approximately 10(6) VV-specific CD4 T cells (approximately 5% of the CD4 T cells) in the spleen. These numbers decreased to approximately 5 x 10(5) CD8 T cells (approximately 5% frequency) and approximately 10(5) CD4 T cells (approximately 0.5% frequency), respectively, by day 30 and were then stably maintained at these levels for >300 days. The size of this VV-specific T-cell response was comparable to that of the T-cell response induced following an acute LCMV infection. (ii) VV-specific CD8 and CD4 T cells were capable of producing gamma interferon (IFN-gamma), tumor necrosis factor alpha (TNF-alpha), and interleukin-2; all cells were able to make IFN-gamma, a subset produced both IFN-gamma and TNF-alpha, and another subset produced all three cytokines. (iii) The CD8 T-cell response to the foreign gene (LCMV NP118-126 epitope) was coordinately regulated with the response to the vector during all three phases (expansion, contraction, and memory) of the T-cell response. The total number of CD8 T cells responding to NP118-126 were approximately 20- to 30-fold lower than the number responding to the VV vector (approximately 1% at the peak and 0.2% in memory). This study provides a better understanding of T-cell immunity induced by VV-based vaccines, and in addition, the technique described in the study can be readily extended to other viral vectors to determine the ratio of the T-cell response to the insert versus the vector. This information will be useful in optimizing prime-boost regimens for vaccination.     

Antiviral immune responses in Itk-deficient mice.

Mice lacking Itk, a T-cell-specific protein tyrosine kinase, have reduced numbers of T cells and reduced responses to allogeneic major histocompatibility molecules. This study analyzed antiviral immune responses in mice deficient for Itk. Primary cytotoxic T-lymphocyte (CTL) responses were analyzed after infection with lymphocytic choriomeningitis virus (LCMV), vaccinia virus (VV), and vesicular stomatitis virus (VSV). Ex vivo CTL activity was consistently reduced by a factor of two to six for the different viruses. CTL responses after restimulation in vitro were similarly reduced unless exogenous cytokines were added. In the presence of interleukin-2 or concanavalin A supernatant, Itk-deficient and control mice responded similarly. Interestingly, while LCMV was completely eliminated by day 8 in both Itk-deficient and control mice, VV cleared from itk-/- mice with delayed kinetics. Antibody responses were evaluated after VSV infection. Both the T-cell-independent neutralizing immunoglobulin M (IgM) and the T-cell-dependent IgG responses were similar in Itk-deficient and control mice. Taken together, the results show that CTL responses are reduced in the absence of Itk whereas antiviral B-cell responses are not affected.