The RiboPuromycylation Method Reveals Rapid IL12-Dependent Antigen-Independent Activation of Bone Marrow Virtual Memory T cells (INC1P.403)

Protein translation is an essential and intricately regulated cellular process that can potentially be used to monitor immune cell activation. We extended the RiboPuromycylation Method (RPM) to measure protein translation in vivo. Unexpectedly, 24 h following either vaccinia virus (VACV) or vesicular stomatitis virus (VSV) infection, 15-30% of bone marrow (BM) CD4+ and CD8+ T cells, all CD44+, increase protein translation. This is days before protein translation is increased in T cells in the draining lymph node or spleen. Bystander BM but not splenic OT-1 T cells demonstrate increased protein translation after VACV infection. Early virus-induced BM T cell translation is absent in IL-12 -/- mice. In BM cultures from naïve mice, addition of IL-12, and especially IL-12 with IL-18 increase T cell translation within hours, with IL1-2 and IL-18 eventually leading to T cell division. Our findings indicate that the BM is a unique environment where bystander T cells are rapidly activated by innate immune response to viruses. We hypothesize that this may allow memory and memory-like T cells to be activated in the absence of cognate antigen to facilitate their functionality when they encounter the cognate pathogen.

The RiboPuroMycylation method reveals surprising protein synthesis in bone marrow T cells within a day of vaccinia virus infection (P3385)

Protein translation is an essential and intricately regulated cellular process. We recently described the RiboPuroMycylation Method (RPM) to measure protein translation in cultured cells. RPM is based on ribosome-catalyzed addition of puromycin to nascent chains followed by detection with a monoclonal antibody specific for puromycin in fixed and permeabilized cells. To apply RPM in vivo, we administered puromycin intravenously and measured puromycin staining via flow cytometry. We were able to detect a robust signal that was blocked by ribosome targeting antibiotics that prevent nascent chain puromycylation. After infection with either vaccinia virus (VACV) or influenza virus, we find profound increases in protein translation in activated T and B cells. In mice with adoptively transferred OT-1 T cells we find that translation of individual cells peaks 3 days after a foot pad infection, i.e. several days before T cell numbers peak. One day after an intra-peritoneal VACV infection, almost 40% of the CD4+ and CD8+ T cells in the bone marrow (BM) have increased protein translation as compared to only 3% of the T cells in the spleen. Using transgenic OT-1 T cells and VACV with or without SIINFEKL, we find that the increased protein translation in the spleen and BM is antigen-dependent. Taken together, these findings illustrate the ability of in vivo RPM to provide novel information regarding the participation of translational regulation in workings of the immune system.

Viral Infection Triggers Rapid Differentiation of Human Blood Monocytes into Dendritic Cells (45.12)

Viruses post a constant threat of human existence. Although human immune system plays a central role in recognizing and eradicating pathogens, how it responds to virus infection remains unclear. Here we show that monocytes make up the majority of a wide variety of virus susceptible population in human peripheral blood mononuclear cells. Notably, infection with influenza virus, vesicular stomatitis virus and vaccinia virus promotes monocytes to differentiate within 18 hours into CD16-negative CD83-positive mature dendritic cells (DCs) with enhanced capacity to activate T cells. Virus-induced monocyte differentiation into DCs does not require cell division, and happens despite the synthesis of virus proteins, suggesting that monocytes thwart the capacity of these highly lytic viruses to hijack host cell biosynthesis pathway. Indeed, virus-induced DC differentiation from monocytes requires infectious virus and viral protein synthesis. Thus, our results demonstrate that monocytes are uniquely susceptible to virus infection among human peripheral blood mononuclear cells, with the likely purpose of differentiating into DCs with enhanced capacity to activate innate and acquired anti-virus immunity.

Naturally processed HLA class I-restricted epitopes inform targets of protective CD8+ T cell-mediated immunity (113.11)

Rational design of CD8+ T cell (TCD8)-based vaccines requires knowledge of the immunogenic and protective epitopes presented during infection, information which is currently lacking. Using the clinically successful smallpox vaccine as a model, ~200 novel naturally processed vaccinia viral peptides presented by HLA-A*0201 and -B*0702 molecules were identified and characterized. Humans showed a variegated response to these determinants, reminiscent of the hierarchic response seen in HLA class I transgenic mice. Importantly, multiple TCD8 epitopes were commonly recognized by humans and mice and identified potential targets for protective TCD8-mediated immunity. After acute infection, both dominant and subdominant TCD8 specificities exhibited all of the immunologic features necessary for protection. However, early and efficient presentation of immune determinants during infection ensured protective responses, regardless of dominance, such that subunit vaccination targeting subdominant or recessive TCD8 specificities conferred protection against lethal poxvirus challenge. Hence, an in-depth knowledge of naturally processed T cell epitopes coupled with the identification of TCD8-based targets that protect from lethal poxviral infection are essential for rational vaccine design.

Factors influencing immunodominance hierarchies in TCD8+ -mediated antiviral responses

CD8(+) T-lymphocytes (T(CD8+)) perform a critical role in immunity against tumors and virus infections. A central feature of T(CD8+) immune responses is immunodominance: the observation that T(CD8+) responses consist of a limited collection of specificities with a structured hierarchy. These immunodominance hierarchies result from a complex combination of factors. Major roles are played by peptide binding affinity, T-cell repertoire, and antigen processing and presentation. While the bulk of our information comes from mouse model systems, an increasing number of human studies suggest that immunodominance will be even more complicated. This review outlines current knowledge of T(CD8+ )immunodominance to viral antigens and discusses the relevance and importance of a thorough understanding for the rational design of vaccines that elicit effective T(CD8+) responses.

Drug Controlled Antigen Degradation Defines Sources of MHC Class I Peptides and Enables Pathway Discovery (130.29)

Defective Ribosomal Products (DRiPs) are an important source of MHC class I peptide ligands. How and where DRiPs are generated is an unanswered question with important implications for rationale design of T cell vaccines. To better understand DRiP antigen processing, we developed a novel antigenic protein, based upon the ProteoTuner technology, whose metabolic stability is controlled by a small molecule termed Shield-1. We show robust MHC class I antigen presentation derived from this shield controlled recombinant antigenic protein (SCRAP) regardless of the overall stability conveyed by shield. We use Shield-1 to generate a large pool of SCRAP, that is rapidly degraded upon Shield-1 removal, and show that these “retirees” are a much less efficient source of peptides than DRiPs. These findings demonstrate that true DRiPs are the source for a large proportion of peptide-MHC class I complexes. We also exploit the unique properties of this system to screen for chemicals and siRNAs that specifically inhibit DRiP antigen presentation, and have generated results demonstrating a role for deubiquitinating enzymes in DRiP antigen presentation.

Expression of the 1918 influenza A virus PB1-F2 enhances the pathogenesis of viral and secondary bacterial pneumonia

Secondary bacterial pneumonia frequently claimed the lives of victims during the devastating 1918 influenza A virus pandemic. Little is known about the viral factors contributing to the lethality of the 1918 pandemic. Here we show that expression of the viral accessory protein PB1-F2 enhances inflammation during primary viral infection of mice and increases both the frequency and severity of secondary bacterial pneumonia. The priming effect of PB1-F2 on bacterial pneumonia could be recapitulated in mice by intranasal delivery of a synthetic peptide derived from the C-terminal portion of the PB1-F2. Relative to its isogenic parent, an influenza virus engineered to express a PB1-F2 with coding changes matching the 1918 pandemic strain was more virulent in mice, induced more pulmonary immunopathology, and led to more severe secondary bacterial pneumonia. These findings help explain both the unparalleled virulence of the 1918 strain and the high incidence of fatal pneumonia during the pandemic.

Restricting expression prolongs expression of foreign genes introduced into animals by retroviruses

If foreign genes are ubiquitously expressed in mice using a viral vector, expression is abrogated by CD8(+) cells in 2 to 4 weeks. However, if the expression of the genes is confined to skeletal muscle cells, the CD8(+) T-cell response is much weaker and expression is maintained for more than 6 weeks. These data show that restricting the expression of foreign genes to skeletal muscle cells and presumably to other cells that are inefficient at antigen presentation can prolong the expression of a foreign gene product.

Vaccination with DNA encoding internal proteins of influenza virus does not require CD8(+) cytotoxic T lymphocytes: either CD4(+) or CD8(+) T cells can promote survival and recovery after challenge

DNA vaccination offers the advantages of viral gene expression within host cells without the risks of infectious virus. Like viral vaccines, DNA vaccines encoding internal influenza virus proteins can induce immunity to conserved epitopes and so may defend the host against a broad range of viral variants. CD8(+) cytotoxic T lymphocytes (CTL) have been described as essential effectors in protection by influenza nucleoprotein (NP), although a lesser role of CD4(+) cells has been reported. We immunized mice with plasmids encoding influenza virus NP and matrix (M). NP + M DNA allowed B6 mice to survive otherwise lethal challenge infection, but did not protect B6-beta(2)m(-/-) mice defective in CD8(+) CTL. However, this does not prove CTL are required, because beta(2)m(-/-) mice have multiple immune abnormalities. We used acute T cell depletion in vivo to identify effectors critical for defense against challenge infection. Since lung lymphocytes are relevant to virus clearance, surface phenotypes and cytolytic activity of lung lymphocytes were analyzed in depleted animals, along with lethal challenge studies. Depletion of either CD4(+) or CD8(+) T cells in NP + M DNA-immunized BALB/c mice during the challenge period did not significantly decrease survival, while simultaneous depletion of CD4(+) and CD8(+) cells or depletion of all CD90(+) cells completely abrogated survival. We conclude that T cell immunity induced by NP + M DNA vaccination is responsible for immune defense, but CD8(+) T cells are not essential in the active response to this vaccination. Either CD4(+) or CD8(+) T cells can promote survival and recovery in the absence of the other subset.

Strategies for tumor elimination by cytotoxic T lymphocytes

Despite differences in their tissue of origin, many tumors share high level expression of certain tumor-associated proteins. Our laboratory has focused on the possibility of utilizing antigenic components of these proteins as a focus for T-cell immunotherapy of cancer. The advantage of targeting such commonly expressed proteins is the fact that such therapy could be of value in eliminating many different types of tumors. A potential barrier in the identification of T-cell epitopes derived from these proteins and presented by tumor cells is the fact that these proteins are also expressed at low levels in some normal tissues, and therefore, self-tolerance may eliminate T cells that are capable of recognizing these epitopes with high avidity. We have utilized two different murine model systems to explore the extent to which self-tolerance may limit the immune response to a tumor-specific antigen. The first compared the ability of mice deficient in expression of murine p53 (p53 knock-out mice) and normal mice, to respond against several epitopes of the p53 protein. The second model compares the ability of conventional mice with transgenic mice that express the influenza hemagglutinin in the periphery to respond to a dominant antigenic peptide of this transgene product. In both models we have investigated the effect self-tolerance has on elimination of tumors expressing the toleragen.

Herpes simplex virus turns off the TAP to evade host immunity

Many viruses have evolved mechanisms to avoid detection by the host immune system. Herpes simplex virus (HSV) expresses an immediate early protein, ICP47, which blocks presentation of viral peptides to MHC class I-restricted cells. The properties of the newly synthesized class I molecules in HSV-infected cells resemble those of cell lines deficient in the transporter associated with antigen processing (TAP) in that class I molecules are retained in the endoplasmic reticulum, and the heavy chain and beta 2-microglobulin subunits dissociate in detergent extracts but the complex can be stabilized by peptides. We show here that ICP47 binds to TAP and prevents peptide translocation into the endoplasmic reticulum.

MHC-encoded proteasome subunits LMP2 and LMP7 are not required for efficient antigen presentation

LMP2 and LMP7 are proteins encoded by MHC genes that are tightly linked to the genes encoding TAP, the transporter that conveys peptides from the cytosol to the endoplasmic reticulum for assembly with MHC class I molecules. LMP2 and LMP7 are subunits of a subset of proteasomes, large molecular assemblies with multi-proteolytic activities believed to degrade damaged and unwanted cellular proteins. Like TAP and class I molecules themselves, expression of LMP genes is enhanced after exposure of cells to IFN-gamma. These findings implicate LMP2 and LMP7 in the cytosolic production of antigenic peptides. Doubts have been cast, however, on the role of LMP2 and LMP7 in Ag processing, because cells lacking these proteins possess class I molecules that contain peptides quantitatively and qualitatively indistinguishable from the peptides bound to class I molecules derived from normal cells. In this paper we show that cells lacking LMP2 and LMP7 present seven TAP-dependent determinants derived from viral proteins. For two determinants, the kinetics of presentation are shown to be similar for LMP-expressing and -nonexpressing cells. We also demonstrate biochemically that peptide is not limiting in the assembly of class I molecules in LMP-nonexpressing cells. These findings provide additional evidence that LMP2 and LMP7 are not required for efficient Ag presentation, and suggest that these proteins have either a more specialized role in the production of class I-associated peptides, or are not involved in the processing of proteins for association with class I molecules.

MHC-encoded proteasome subunits LMP2 and LMP7 are not required for efficient antigen presentation

LMP2 and LMP7 are proteins encoded by MHC genes that are tightly linked to the genes encoding TAP, the transporter that conveys peptides from the cytosol to the endoplasmic reticulum for assembly with MHC class I molecules. LMP2 and LMP7 are subunits of a subset of proteasomes, large molecular assemblies with multi-proteolytic activities believed to degrade damaged and unwanted cellular proteins. Like TAP and class I molecules themselves, expression of LMP genes is enhanced after exposure of cells to IFN-gamma. These findings implicate LMP2 and LMP7 in the cytosolic production of antigenic peptides. Doubts have been cast, however, on the role of LMP2 and LMP7 in Ag processing, because cells lacking these proteins possess class I molecules that contain peptides quantitatively and qualitatively indistinguishable from the peptides bound to class I molecules derived from normal cells. In this paper we show that cells lacking LMP2 and LMP7 present seven TAP-dependent determinants derived from viral proteins. For two determinants, the kinetics of presentation are shown to be similar for LMP-expressing and -nonexpressing cells. We also demonstrate biochemically that peptide is not limiting in the assembly of class I molecules in LMP-nonexpressing cells. These findings provide additional evidence that LMP2 and LMP7 are not required for efficient Ag presentation, and suggest that these proteins have either a more specialized role in the production of class I-associated peptides, or are not involved in the processing of proteins for association with class I molecules.

RMA/S cells present endogenously synthesized cytosolic proteins to class I-restricted cytotoxic T lymphocytes

RMA/S is a mutant cell line with decreased cell surface expression of major histocompatibility complex class I molecules that has been reported to be deficient in presenting endogenously synthesized influenza virus nucleoprotein (NP) to cytotoxic T lymphocytes (CTL). In the present study we show that RMA/S cells can present vesicular stomatitis virus nucleocapsid protein, and, under some conditions, NP, to Kb-and Db-restricted CTL, respectively. Antigen presentation results from processing of cytosolic pools of endogenously synthesized proteins, and not the binding to cell surface class I molecules of antigenic peptides present in the virus inoculum or released from infected cells. Antigen processing of RMA/S differs, however, from processing by wild-type cells in requiring greater amounts of antigen, longer times to assemble or transport class I-peptide complexes, and in being more sensitive to blocking by anti-CD8 antibody. Thus, the antigen processing deficit in RMA/S cells is of a partial rather than absolute nature.

Murine cell lines stably expressing the influenza virus hemagglutinin gene introduced by a recombinant retrovirus vector are constitutive targets for MHC class I- and class II-restricted T lymphocytes

A retrovirus vector containing the hemagglutinin (HA) gene of influenza virus was constructed and used to infect murine cell lines of fibroblast, mastocytoma and B cell lineages which are able to present antigens to MHC-restricted T cells. Stable cell lines were selected in which the retrovirus vector integrated as a single copy in almost all of the individual cell clones examined. The HA mRNA was shown to be of the expected length by Northern blot analysis, but the levels varied among the cell clones. Although the HA transcript was difficult to detect in any of the retrovirus-infected cell clones derived from fibroblasts, HA Ag was easily detected on the cell surface by cytofluorographic analysis. Significantly, retrovirus-infected clones derived from each cell type were recognized by HA-specific class I and class II MHC-restricted T lymphocytes. HA produced in these cells was able to be acquired, processed, and presented to class II-restricted T cells by additional, non-HA-expressing APC. This indicates that HA endogenously synthesized within these cell lines is available for Ag processing by an exogenous route.

Murine cell lines stably expressing the influenza virus hemagglutinin gene introduced by a recombinant retrovirus vector are constitutive targets for MHC class I- and class II-restricted T lymphocytes

A retrovirus vector containing the hemagglutinin (HA) gene of influenza virus was constructed and used to infect murine cell lines of fibroblast, mastocytoma and B cell lineages which are able to present antigens to MHC-restricted T cells. Stable cell lines were selected in which the retrovirus vector integrated as a single copy in almost all of the individual cell clones examined. The HA mRNA was shown to be of the expected length by Northern blot analysis, but the levels varied among the cell clones. Although the HA transcript was difficult to detect in any of the retrovirus-infected cell clones derived from fibroblasts, HA Ag was easily detected on the cell surface by cytofluorographic analysis. Significantly, retrovirus-infected clones derived from each cell type were recognized by HA-specific class I and class II MHC-restricted T lymphocytes. HA produced in these cells was able to be acquired, processed, and presented to class II-restricted T cells by additional, non-HA-expressing APC. This indicates that HA endogenously synthesized within these cell lines is available for Ag processing by an exogenous route.

Target and effector cell fusion accounts for B lymphocyte-mediated lysis of mouse hepatitis virus-infected cells

In confirmation of previous reports, we observed lysis of mouse hepatitis virus (MHV)-infected target cells in the presence of spleen and lymph node cells from nonimmunized mice possessing a B cell surface phenotype (IgM+, IgG+, J11d+, Ia+, Fc+, Thy1-, MAC-1- and asialo-GM1-). Lysis was inhibited by MHV-specific antisera. The presence of immunoglobulin at the surface of B cells is not required for cytolysis since MHV-infected target cells are lysed in the presence of the B cell hybridoma Sp2/0, which fails to synthesize immunoglobulin. Using 51Cr-labelled Sp2/0 cells, both target and effector cells were shown to undergo cytolysis. Direct observation of target and effector cells co-incubated after labelling with different fluorescent dyes demonstrated that lysis correlates with the fusion of B cells and MHV-infected cells. These findings are consistent with the idea that the E2 protein of MHV, which is expressed on the infected cell surface and has receptor and membrane-fusion activities at neutral pH, selectively mediates fusion with cells of B lymphocyte lineage. This may represent a general mechanism by which enveloped viruses with fusion proteins that function at neutral pH can interfere with the function of subsets of immune cells.

Selection of the T-cell repertoire during ontogeny

This report examines conflicting hypotheses concerning T-cell repertoire selection in terms of H-2 restriction during ontogeny. The experiments described in this report were incompatible with the hypothesis that bias in the repertoire is solely a consequence of "more or less intentional priming" by foreign antigen. Rather, results indicate that the repertoire is selected by self major histocompatibility complex (MHC) antigens in the absence of foreign antigens. Allorestricted T cells, the existence of which was previously thought to be incompatible with the concept of complete repertoire selection by self MHC antigens, were shown to significantly cross-react on targets expressing self MHC antigens. Thus, it is possible that allorestricted T cells are simply cross-reactive T-cell clones restricted by self MHC antigens; indeed, all experimental data were compatible with the idea of complete selection of the T-cell repertoire in terms of H-2 restriction by self MHC antigens.

Selection of the T cell repertoire during ontogeny: limiting dilution analysis

The relative frequencies of cytotoxic T lymphocyte precursors (CTL-P) specific for minor histocompatibility antigens that are restricted to either k or b major histocompatibility complex antigens in normal B6 mice and in B6 leads to (B6 x CBA)F1 radiation chimeras were estimated. The ratio of k-restricted to b-restricted CTL-P determined for the radiation chimeras served as a base value to which the ratio of k: b-restricted CTL from normal B6 mice can be compared. The frequencies of allorestricted CTL-P in B6 mice were determined after the B6 cells had been depleted of k-reactive cells by filtration through irradiated B6.H-2k mice. Lymphocytes from immunized animals were used to obtain frequency estimates of CTL-P specific for minor H antigens. The cultures contained nu/nu spleen cells syngeneic to the responder, but no exogenous interleukin 2 (T cell growth factor) since a high degree of nonspecific killing was found in IL2-supplemented cultures. It was found that the CTL-P frequency of B6 mice to BALB.B antigens in the lymph node cells of normal or immune B6 mice were 1 in 300 000 and 1 in 8000, respectively. In B6 leads to (B6 x CBA)F1 radiation chimeras, the ratio of BALB.B/BALB.K-specific CTL-P was 3:1; this ratio was 17:1 in normal B6 mice. Twenty-five percent of CTL clones from normal B6 mice lysing BALB.K target also lysed BALB.B target whereas less than 1% of CTL clones lytic on BALB.K target from the chimeras showed this cross-reaction The results are compatible with the idea of complete positive selection of self MHC-restricted CTL-P in the absence of priming effects by foreign antigen.

The frequency of mutant-specific killer precursors in related and unrelated mouse strains

The frequency of mutant-specific cytotoxic T lymphocyte precursors (CTL-P) has been analyzed in several mouse strains. While H-2 antigens of responder strains have a limited influence on the frequency of major H antigen-specific CTL-P, the frequency of mutant-specific CTL-P can be equally as high in unrelated and related strains. In general, the frequency does not reflect the closeness of kinship between stimulator and responder.

Influenzal pneumonia: early appearance of cross-reactive T cells in lungs of mice primed with heterologous type A viruses

Mice were first primed with a type A or a type B influenza virus and then challenged intranasally at least 1 month later with another type A virus. Potent cytotoxic T cell populations were found in lung, and effector T cell function was also demonstrated in blood, spleen and mediastinal lymph nodes. Lymphocytes isolated from all of these anatomical sites were active against target cells infected with the same, or with serologically different, type A influenza viruses. Also, prior exposure to another type A virus resulted in more rapid development of effector function than was seen in mice that had first been infected with B/Lee. Cytotoxic T cell populations generated in mice with influenza thus tend overall to be type-specific, and there is substantial localization of these effector lymphocytes in the pneumonic lung.

Induction of virus-specific modifications recognized by cytotoxic T cells is not altered by prior substitution of target cells with trinitrophenol

Cytotoxic thymus-derived lymphocytes generated after interaction with trinitrophenyl (TNP)-substituted or virus-infected cells only lyse H-2 compatible target cells modified with the component used to immunize (TNP or virus). Prior saturation of TNP-reactive sites inhibits neither the infectivity of influenza A viruses, nor the capacity of infected cells to develop antigenic changes recognized by influenza-immune T cells. The two antigens are distinct entities on the cell membrane and do not obviously compete to form interactions with H-2 molecules.

Heterogeneity of the cytotoxic response of thymus-derived lymphocytes after immunization with influenza viruses

Immunization of mice with serologically distinct type A influenza viruses results in development of highly crossreactive populations of cytotoxic thymus-derived lymphocytes (T cells). This can be shown both at the level of effector function and of priming for an enhanced response after challenge with another type A virus. Cytotoxic activity is demonstrable, in both the primary and secondary situations, only for H-2 compatible interactions. Further analysis by competitive inhibition experiments indicates that some of the T cell clones generated are specific for the virus used to immunize, while others are much less restricted. Secondary stimulation may result in preferential stimulation of the crossreactive T cells if the type A viruses used are very different serologically. When more closely related viruses are used, however, some degree of specificity is seen for the challenge virus. Even so, the patterns of crossreactivity observed are complex, and cannot be readily predicted on the basis of known serological relationships between surface hemagglutinin and neuraminidase antigens of type A influenza viruses.

Generation of both cross-reactive and virus-specific T-cell populations after immunization with serologically distinct influenza A viruses

Specificity of cytotoxic T-cell function was investigated for a range of different influenza viruses. T cells from mice immunized with A or B strain influenza viruses, or with vaccinia virus, showed reciprocal exclusion of cytotoxicity. Extensive cross-reactivity was, however, found for lymphocyte populations from mice infected with a variety of serologically distinct influenza A viruses, though serum antibodies did not cross-react when tested in a radioimmunoassay using comparable target cells as immunoadsorbents. This apparent lack of T-cell specificity was recognized for immune spleen cells generated after intraperitoneal inoculation of high titers of virus, and for mediastinal lymph node populations from mice with pneumonia due to infection with much less virus. The phenomenon could not be explained on the basis of exposure to the chicken host component, which is common to A and B strain viruses. However, not all of the virus-immune T-cell clones are cross-reactive. Competitive-inhibition experiments indicate that a considerable proportion of the lymphocyte response is restricted to the immunizing virus. Even so, the less specific component is significant. Also, exposure to one type A virus was found to prime for an enhanced cell-mediated immunity response after challenge with a second, serologically different A strain virus.