Specific epitope-induced conversion of CD8+ memory cells into effector cytotoxic T lymphocytes in vitro: presentation of peptide antigen by CD8+ T cells

Steric recognition of T-cell receptor contact residues is required to map mutant epitopes by immunoinformatical programmes

MHC class I-restricted CD8 T-lymphocyte epitopes comprise anchor motifs, T-cell receptor (TCR) contact residues and the peptide backbone. Serial variant epitopes with substitution of amino acids at either anchor motifs or TCR contact residues have been synthesized for specific interferon-γ responses to clarify the TCR recognition mechanism as well as to assess the epitope prediction capacity of immunoinformatical programmes. CD8 T lymphocytes recognise the steric configuration of functional groups at the TCR contact side chain with a parallel observation that peptide backbones of various epitopes adapt to the conserved conformation upon binding to the same MHC class I molecule. Variant epitopes with amino acid substitutions at the TCR contact site are not recognised by specific CD8 T lymphocytes without compromising their binding capacity to MHC class I molecules, which demonstrates two discrete antigen presentation events for the binding of peptides to MHC class I molecules and for TCR recognition. The predicted outcome of immunoinformatical programmes is not consistent with the results of epitope identification by laboratory experiments in the absence of information on the interaction with TCR contact residues. Immunoinformatical programmes based on the binding affinity to MHC class I molecules are not sufficient for the accurate prediction of CD8 T-lymphocyte epitopes. The predictive capacity is further improved to distinguish mutant epitopes from the non-mutated epitopes if the peptide-TCR interface is integrated into the computing simulation programme.

Regulation of adaptive immunity by natural killer cells

Natural killer (NK) cells are well recognized as cytolytic effector cells of the innate immune system. In the past several years, the structure and function of NK cell receptors for the major histocompatibility complex (MHC) class I molecules and other ligands have been the subject of extensive studies. These studies. These studies have focused largely on the mechanisms of target cell recognition for lysis. Another aspect of NK cell function that seems to be underappreciated is their role in immune regulation. Since NK cells produce a number of immunologically relevant cytokines, it has been suggested that these cells may modulate the development of the adaptive immune response. But, is it the only mechanism by which NK cells interact with cells involved in the induction of antigen-specific responses? This article reviews some older and more recent studies and attempts to place NK cells in the context of potent immune regulators of T cell responses.

Activation of influenza-specific memory cytotoxic T lymphocytes by Concanavalin A stimulation

Traditionally, the in vitro activation of virus-specific memory cytotoxic T lymphocytes (CTLs) has been achieved by stimulating the CTLs with antigen-presenting cells (APCs) infected with an appropriate virus or pulsed with virus-specific antigenic peptides. Here, we describe the utilization of the polyclonal activator Concanavalin A (ConA) for in vitro restimulation of memory CTLs from virus-primed mice. Using this simple method, the activation of splenocytes with ConA for 3 days (i) eliminates the need to stimulate with virus-pulsed APCs and (ii) generates CD8+ CTLs that exhibit virus specificity and MHC-restricted lytic activity similar to CTLs obtained by conventional viral restimulation. In vitro ConA stimulation of splenocytes from BALB/c mice primed with the A/Texas/77 or A/Japanese/57 strain of influenza virus and from C57L/J mice infected with the A/Texas strain, generated CTLs with specific lytic activity. Hence reactivation of memory CTLs by this method is a general phenomenon rather than a mouse or viral strain-specific one. The ConA stimulation method used here had a recall of long-term (1 year) memory CTLs that effectively lysed virally infected targets. Further ConA-stimulated effector lymphocytes from virally primed animals have been shown to recognize and subsequently lyse target cells pulsed with virus or virus-derived peptides. The ConA reactivation of specific anti-viral CTLs may facilitate (i) studying anti-viral CTL responses and (ii) identifying of viral epitopes when unknown or when appropriate viral stimulation is impossible.

Recruitment, activation and proliferation of CD8+ memory T cells in an immunoprivileged site

The capacity of a memory cytotoxic T lymphocyte (CTL) population to protect against viral infections is well established, but the processes underlying this protection are less well understood. We have used heterotypic intranasal immunization with influenza A/X31 (H3N2) to protect against a subsequent infection with the neurovirulent influenza A/WSN (H1N1) in either the cerebrospinal fluid or the immunoprivileged brain parenchyma. Viral clearance from both sites was associated with a local infiltration and proliferation of A/WSN-specific CD8+ T cells. Infection in the cerebrospinal fluid elicited a proliferative response in the draining lymph nodes, an anti-H1N1 serum antibody response and an increase in the extracerebral A/WSN-specific CTL precursor frequency. In contrast, infection in the brain parenchyma elicited no lymph node proliferative response or serum antibody response and caused a transient decrease in the extracerebral CTL precursor frequency. Thus the memory CTL population protected against an intracerebral viral infection independent of any immune response occurring in systemic lymphoid tissue.

Protection against influenza virus encephalitis by adoptive lymphocyte transfer

CD8+ cytotoxic T lymphocytes (CTL) have an established role in anti-viral immunity, but whether CTL function efficiently in the brain remains unclear. In particular, virus-infected neurons, which express only low levels of MHC class I antigens and are resistant to the induction of apoptosis, could constitute a relatively intractable CTL target. We have used immune lymphocytes adoptively transferred into the CSF to protect naive mice against an intracerebral infection with influenza A/WSN, a virus that infects neurons in the brain parenchyma and causes a lethal encephalitis. After in vitro restimulation, heterotypically immune spleen cells protected against A/WSN encephalitis in an H-2-restricted, CD8-dependent, CD4-independent manner. Adoptively transferred CTL clones were also protective. Homotypically immune spleen cells additionally mediated CD8-independent, H-2-unrestricted protection, probably due to the generation of A/WSN-specific plasma cells from memory B cells during in vitro restimulation. Thus after in vitro restimulation, either CTL or B cells adoptively transferred into the CSF protected against an acutely lethal intracerebral virus infection.

The generation of memory antigen-specific cytotoxic T cell responses by CD28/CD80 interactions in the absence of antigen

The interaction of co-stimulatory molecules CD80/CD86 on antigen-presenting cells with CD28 on naive CD8+ cytotoxic T (Tc) cells is understood to be critical in the induction of Tc effectors. CD80 is capable of providing signal 2 for the activation of Tc cells, but has no effect if encountered in the absence of specific peptide/MHC complexes (signal 1). We have found that CD80 presented in vitro to resting memory viral-immune or alloimmune Tc cells can provide sufficient stimulus for the generation of effector Tc cells in the absence of specific antigen, the peptide/MHC class I complex. Effector Tc cells generated in vitro from influenza- or class I alloantigen-primed mice by co-stimulation in the absence of antigen require exogenous interleukin (IL)-2 signaling via the cell surface-expressed IL-2 receptor or, under conditions of IL-2 blockade, exogenous IL-7. Activation of memory Tc cells by signal 1 and 2 is independent of IL-2 and IL-7. Although memory influenza-immune Tc cells did respond to CD80 in the absence of antigen, the presence of antigen +CD80 enabled an earlier induction of these Tc cells and they retained their lytic activity in vitro over a longer time period. The capacity of memory Tc cells to be activated by signal 2 alone provides one explanation for the observed heterogeneity of phenotype of memory T cells in vivo and a possible mechanism for the maintenence of memory in the absence of persisting antigen.

Aspects of cytotoxic T cell memory

Immunological T cell memory manifest itself in an accelerated second-set graft, or allogeneic tumour cell, rejection. Memory viral-immune cytotoxic T cells have shortened kinetics of induction in vivo and differentiate into more potent effector cells in vitro. The requirements for induction of memory T cells are less stringent than for naive T cells. Memory T cells can be activated by antigen (signal 1) or interaction with co-stimulatory molecules (CD28/CD80, signal 2) alone. Memory T cells are phenotypically distinguishable from naive T cells by a number of cell surface markers, but not from activated T cells. Persistence of antigen is not required for the maintenance of long-lived memory. Continuous stimulation by signal 2 alone and or longevity is sufficient to explain life-long persistence of T cell memory. All available data on memory T cells are consistent with a deterministic model of T cell memory formation, following a precise pathway of T cell differentiation.

Delivery of a viral antigen to the class I processing and presentation pathway by Listeria monocytogenes

Listeria monocytogenes is a facultative intracellular pathogen that grows in the cytoplasm of infected host cells. We examined the capacity of L. monocytogenes to introduce influenza nucleoprotein (NP) into the class I pathway of antigen presentation both in vitro and in vivo. Recombinant L. monocytogenes secreting a fusion of listeriolysin O and NP (LLO-NP) targeted infected cells for lysis by NP-specific class I-restricted cytotoxic T cells. Antigen presentation occurred in the context of three different class I haplotypes in vitro. A hemolysin-negative L. monocytogenes strain expressing LLO-NP was able to present in a class II-restricted manner. However, it failed to target infected cells for lysis by CD8+ T cells, indicating that hemolysin-dependent bacterial escape from the vacuole is necessary for class I presentation in vitro. Immunization of mice with a recombinant L. monocytogenes strain that stably expressed and secreted LLO-NP induced NP-specific CD8+ cytotoxic T lymphocytes. These studies have implications for the use of L. monocytogenes to deliver potentially any antigen to the class I pathway in vivo.

Interleukin 2-independent interleukin 7 activity enhances cytotoxic immune response of HIV-1-infected individuals

Virus-specific cytotoxic T lymphocytes (CTLs), which kill virus-infected cells, are thought to be a major host defense against viral infections. The addition of interleukin 7 (IL-7) at the onset of mitogen-stimulated cultures resulted in a marked (up to threefold) augmentation of env-specific cytotoxicity in human immunodeficiency virus type 1 (HIV-1)-infected individuals (p < 0.001). Addition of IL-7 on day 3 or 5 produced a significant but lesser augmentation of CTL response as compared to day 0. The IL-7-induced proliferative response and augmentation of cytotoxic activity was time and dose dependent, with an optimal IL-7 concentration of 1000 U/ml. Cell surface phenotypic analysis of CTL effector cells indicates that IL-7 primarily affects the proliferation of CD8+ T cells. Anti-IL-2 monoclonal antibody (MAb) substantially inhibited the proliferative effect of IL-2, but did not affect the proliferative effect of IL-7. Endogenous IL-2-induced generation of cytotoxic T cells was blocked by MAbs to IL-2 or IL-2R. The addition of IL-7 restored the process of conversion of precursor CTLs (pCTLs) to mature CTLs (mCTLs) and significantly enhanced specific cytolytic activity. It appears that IL-7 is a potent regulatory cytokine capable of acting independently of IL-2 in mitogen-specific activation of pCTLs to mCTLs. These data suggest that IL-7 should be considered as a potential therapeutic approach in AIDS and other infectious diseases in which CTL response declines.

In vivo administration of major histocompatibility complex class I-specific peptides from influenza virus induces specific cytotoxic T cell hyporesponsiveness

We have been investigating the immunogenicity of two class I major histocompatibility complex-specific peptides with a sequence derived from influenza virus nucleoprotein specific for Kd and one for Db. Peptide-modified splenocytes are unable to immunize for a primary cytotoxic T (Tc) cell response in vivo, or secondary response in vitro. Peptide-modified stimulator cells can boost virus-primed splenocytes for a strong secondary response in vitro. Animals primed with syngeneic peptide-modified splenocytes upon challenge with virus in vivo do not generate strong secondary Tc cell responses on day 3 after challenge in contrast to virus primed animals. Day 6 responses of virus-challenged, peptide-primed animals are reduced as compared to unprimed mice. This hyporesponsiveness is independent of CD8+ T cells in the priming population and can be elicited with tumor cell lines. The data are discussed in the framework of the two-signal model of immune induction.

Transfer of CD8+ T cells into SCID mice and activation of memory virus-specific cytotoxic T cells

The requirements for the activation of naive and memory CD8+ cytotoxic T (Tc) cells into effector virus-specific Tc cells after transferring them into SCID mice were investigated. SCID mice reconstituted with splenocytes or purified CD8+ T cells from naive or influenza-immune syngeneic mice and immunized with influenza virus generated effector Tc cells specific for influenza virus-infected target cells in vitro. The kinetics of the response varied between those two populations. The generation of effector Tc cells after transfer of memory CD8+ T cells indicates that there exists no absolute requirement for 'help' in the activation of memory virus-immune T cells. However, under the conditions described here the in vitro immunogenic peptide NPP derived from influenza nucleoprotein is not sufficient to elicit a response in vivo.

Induction of primary anti-viral cytotoxic T cells by in vitro stimulation with short synthetic peptide and interleukin-7

The present study investigated whether a short synthetic peptide NPP, with a modified sequence (147-158 R156-) derived from influenza A virus nucleoprotein with high affinity for Kd major histocompatibility complex class I molecules, could induce primary influenza virus-specific cytotoxic T (Tc) cells in vitro. Naive BALB/c (H-2d) splenocytes did not respond to the stimulation with only NPP with the generation of effector Tc cells specific for influenza A virus-infected target cells in vitro. However, they were able to do so if cultured with NPP in the presence of IL-7. IL-7 activity in this system differed significantly from IL-2 activity in the specificity of the effect. The use of exogenous IL-2, instead of IL-7, with NPP resulted in the induction of lytic cells that lysed both influenza virus-infected and uninfected syngeneic target cells. These results suggest that IL-7 is a potent regulatory cytokine in the antigen-specific activation of resting naive Tc cell precursors and may provide the necessary conditions for the induction of human primary Tc cells in vitro.

Enhancement of antigen-specific activation of CD8+ memory cytotoxic T cells by B cell-derived factors

Purified CD8+ T cells from influenza A/WSN-immune BALB/c (H-2d) mice respond with the generation of secondary A/WSN-specific Tc cells in vitro when stimulated with a synthetic peptide (NPP) with a sequence derived from influenza A virus nucleoprotein with high affinity for Kd class I MHC molecules. The process of the conversion of NPP-Kd-responding Tc cell precursors into effector Tc cells in a population of CD8+ T cells occurs with no demonstrable requirements for accessory cells or their lymphokine products. The addition of culture supernatants from several mouse and human B cell lymphomas and LPS-activated normal mouse B cells to the culture of NPP-stimulated immune CD8+ T cells enhanced the induction of secondary Ag-specific Tc cells. None of the tested supernatants in the absence of Ag (NPP) induced cytolytic Tc cells, indicating that B cell-derived secretory factors can exert their activity only on Ag-exposed CD8+ T cells. The augmentatory effect of these supernatants on Ag-specific activation of memory CD8+ T cells was attributed to the synergism between B cell-derived factors and IL-2 which is produced endogenously in cultures of NPP-stimulated D8+ T cells. The possible role of B cell-derived helper factors is discussed.