Murine model of cytomegalovirus latency and reactivation

Efficient resolution of acute cytopathogenic cytomegalovirus infection through innate and adaptive host immune mechanisms is followed by lifelong maintenance of the viral genome in host tissues in a state of replicative latency, which is interrupted by episodes of virus reactivation for transmission. The establishment of latency is the result of aeons of co-evolution of cytomegaloviruses and their respective host species. Genetic adaptation of a particular cytomegalovirus to its specific host is reflected by private gene families not found in other members of the cytomegalovirus group, whereas basic functions of the viral replicative cycle are encoded by public gene families shared between different cytomegaloviruses or even with herpesviruses in general. Private genes include genes coding for immunoevasins, a group of glycoproteins specifically dedicated to dampen recognition by the host's innate and adaptive immune surveillance to protect the virus against elimination. Recent data in the mouse model of cytomegalovirus latency have indicated that viral replicative latency established in the immunocompetent host is a dynamic state characterized by episodes of viral gene desilencing and immune sensing of reactivated presentation of antigenic peptides at immunological checkpoints by CD8 T cells. This sensing maintains viral replicative latency by triggering antiviral effector functions that terminate the viral gene expression program before infectious viral progeny are assembled. According to the immune sensing hypothesis of latency control, immunological checkpoints are unique for each infected individual in reflection of host MHC (HLA) polymorphism and the proteome(s) of the viral variant(s) harbored in latency.

Experimental preemptive immunotherapy of murine cytomegalovirus disease with CD8 T-cell lines specific for ppM83 and pM84, the two homologs of human cytomegalovirus tegument protein ppUL83 (pp65)

CD8 T cells are the principal antiviral effectors controlling cytomegalovirus (CMV) infection. For human CMV, the virion tegument protein ppUL83 (pp65) has been identified as a source of immunodominant peptides and is regarded as a candidate for cytoimmunotherapy and vaccination. Two sequence homologs of ppUL83 are known for murine CMV, namely the virion protein ppM83 (pp105) expressed late in the viral replication cycle and the nonstructural protein pM84 (p65) expressed in the early phase. Here we show that ppM83, unlike ppUL83, is not delivered into the antigen presentation pathway after virus penetration before or in absence of viral gene expression, while other virion proteins of murine CMV are processed along this route. In cytokine secretion-based assays, ppM83 and pM84 appeared to barely contribute to the acute immune response and to immunological memory. Specifically, the frequencies of M83 and M84 peptide-specific CD8 T cells were low and undetectable, respectively. Nonetheless, in a murine model of cytoimmunotherapy of lethal CMV disease, M83 and M84 peptide-specific cytolytic T-cell lines proved to be highly efficient in resolving productive infection in multiple organs of cell transfer recipients. These findings demonstrate that proteins which fail to prime a quantitatively dominant immune response can nevertheless represent relevant antigens in the effector phase. We conclude that quantitative and qualitative immunodominance are not necessarily correlated. As a consequence of these findings, there is no longer a rationale for considering T-cell abundance as the key criterion for choosing specificities to be included in immunotherapy and immunoprophylaxis of CMV disease and of viral infections in general.

Random, asynchronous, and asymmetric transcriptional activity of enhancer-flanking major immediate-early genes ie1/3 and ie2 during murine cytomegalovirus latency in the lungs

The lungs are a major organ site of cytomegalovirus (CMV) pathogenesis, latency, and recurrence. Previous work on murine CMV latency has documented a high load and an even distribution of viral genomes in the lungs after the resolution of productive infection. Initiation of the productive cycle requires expression of the ie1/3 transcription unit, which is driven by the immediate-early (IE) promoter P(1/3) and generates IE1 and IE3 transcripts by differential splicing. Latency is molecularly defined by the absence of IE3 transcripts specifying the essential transactivator protein IE3. In contrast, IE1 transcripts were found to be generated focally and randomly, reflecting sporadic P(1/3) activity. Selective generation of IE1 transcripts implies molecular control of latency operating after ie1/3 transcription initiation. P(1/3) is regulated by an upstream enhancer. It is widely assumed that the viral transcriptional program is started by activation of the enhancer through the binding of transcription factors. Accordingly, stochastic transcription during latency might reflect episodes of enhancer activation by the "noise" activity of intrinsic transcription factors. In addition to ie1/3, the enhancer controls gene ie2, which has its own promoter, P(2), and is transcribed in opposite direction. We show here that ie2 is also randomly transcribed during latency. Notably, however, ie1 and ie2 were found to be expressed independently. We infer from this finding that expression of the major IE genes is regulated asymmetrically and asynchronously via the combined control unit P(1/3) -E-P(2). Our data are consistent with a stochastic nature of enhancer action as it is proposed by the "binary" or probability model.

Enrichment of immediate-early 1 (m123/pp89) peptide-specific CD8 T cells in a pulmonary CD62L(lo) memory-effector cell pool during latent murine cytomegalovirus infection of the lungs

Interstitial cytomegalovirus (CMV) pneumonia is a clinically relevant complication in recipients of bone marrow transplantation (BMT). Recent data for a model of experimental syngeneic BMT and concomitant infection of BALB/c mice with murine CMV (mCMV) have documented the persistence of tissue-resident CD8 T cells after clearance of productive infection of the lungs (J. Podlech, R. Holtappels, M.-F. Pahl-Seibert, H.-P. Steffens, and M. J. Reddehase, J. Virol. 74:7496-7507, 2000). It was proposed that these cells represent antiviral "standby" memory cells whose functional role might be to help prevent reactivation of latent virus. The pool of pulmonary CD8 T cells was composed of two subsets defined by the T-cell activation marker L-selectin (CD62L): a CD62L(hi) subset of quiescent memory cells, and a CD62L(lo) subset of recently resensitized memory-effector cells. In this study, we have continued this line of investigation by quantitating CD8 T cells specific for the three currently published antigenic peptides of mCMV: peptide YPHFMPTNL processed from the immediate-early protein IE1 (pp89), and peptides YGPSLYRRF and AYAGLFTPL, derived from the early proteins m04 (gp34) and M84 (p65), respectively. IE1-specific CD8 T cells dominated in acute-phase pulmonary infiltrates and were selectively enriched in latently infected lungs. Notably, most IE1-specific CD8 T cells were found to belong to the CD62L(lo) subset representing memory-effector cells. This finding is in accordance with the interpretation that IE1-specific CD8 T cells are frequently resensitized during latent infection of the lungs and may thus be involved in the maintenance of mCMV latency.

Murine model of interstitial cytomegalovirus pneumonia in syngeneic bone marrow transplantation: persistence of protective pulmonary CD8-T-cell infiltrates after clearance of acute infection

Interstitial pneumonia (IP) is a severe organ manifestation of cytomegalovirus (CMV) disease in the immunocompromised host, in particular in recipients of bone marrow transplantation (BMT). Diagnostic criteria for the definition of CMV-IP include clinical evidence of pneumonia together with CMV detected in bronchoalveolar lavage or lung biopsy. We have used the model of syngeneic BMT and simultaneous infection of BALB/c mice with murine CMV for studying the pathogenesis of CMV-IP by controlled longitudinal analysis. A disseminated cytopathic infection of the lungs with fatal outcome was observed only when reconstituting CD8 T cells were depleted. Neither CD8 nor CD4 T cells mediated an immunopathogenesis of acute CMV-IP. By contrast, after efficient hematolymphopoietic reconstitution, viral replication in the lungs was moderate and focal. The histopathological picture was dominated by preferential infiltration of CD8 T cells confining viral replication to inflammatory foci. Notably, after clearance of acute infection, CD62L(lo) and CD62L(hi) subsets of CD44(+) memory CD8 T cells were found to persist in lung tissue. One can thus operationally distinguish an early CMV-positive IP (phase 1) and a late CMV-negative IP (phase 2). According to the definition, phase 2 histopathology would not be diagnosed as a CMV-IP and could instead be misinterpreted as a CMV-induced immunopathology. We document here that phase 1 as well as phase 2 pulmonary CD8 T cells are capable of exerting effector functions and are effectual in protecting against productive infection. We propose that antiviral "stand-by" memory-effector T cells persist in the lungs to prevent virus recurrence from latency.

The immunogenicity of human and murine cytomegaloviruses

Cytomegaloviruses are strictly host-species-specific. During an aeon of co-evolution, virus and host have found an arrangement: the productive and cytopathogenic cycle of viral gene expression is held in check by the host's immune response. As a consequence, cytomegalovirus disease is restricted to the immunocompromised host. The virus has evolved strategies to avoid its elimination and eventually hides itself in a silent state, referred to as 'viral latency'. Redundant molecular mechanisms have been identified by which cytomegaloviruses interfere with antigen presentation pathways to 'evade' immune control. In the annual period covered by this review, the IE1 protein was revisited as an immunodominant antigen of human cytomegalovirus and the identification of a first antigenic early-phase peptide of murine cytomegalovirus that escapes viral immunosubversive mechanisms may initiate a period of research on the immune control of cytomegaloviruses 'beyond immune evasion'.

The putative natural killer decoy early gene m04 (gp34) of murine cytomegalovirus encodes an antigenic peptide recognized by protective antiviral CD8 T cells

Several early genes of murine cytomegalovirus (MCMV) encode proteins that mediate immune evasion by interference with the major histocompatibility complex class I (MHC-I) pathway of antigen presentation to cytolytic T lymphocytes (CTL). Specifically, the m152 gene product gp37/40 causes retention of MHC-I molecules in the endoplasmic reticulum (ER)-Golgi intermediate compartment. Lack of MHC-I on the cell surface should activate natural killer (NK) cells recognizing the "missing self." The retention, however, is counteracted by the m04 early gene product gp34, which binds to folded MHC-I molecules in the ER and directs the complex to the cell surface. It was thus speculated that gp34 might serve to silence NK cells and thereby complete the immune evasion of MCMV. In light of these current views, we provide here results demonstrating an in vivo role for gp34 in protective antiviral immunity. We have identified an antigenic nonapeptide derived from gp34 and presented by the MHC-I molecule D(d). Besides the immunodominant immediate-early nonapeptide consisting of IE1 amino acids 168-176 (IE1(168-176)), the early nonapeptide m04(243-251) is the second antigenic peptide described for MCMV. The primary immune response to MCMV generates significant m04-specific CD8 T-cell memory. Upon adoptive transfer into immunodeficient recipients, an m04-specific CTL line controls MCMV infection with an efficacy comparable to that of an IE1-specific CTL line. Thus, gp34 is the first noted early protein of MCMV that escapes viral immune evasion mechanisms. These data document that MCMV is held in check by a redundance of protective CD8 T cells recognizing antigenic peptides in different phases of viral gene expression.

Patchwork pattern of transcriptional reactivation in the lungs indicates sequential checkpoints in the transition from murine cytomegalovirus latency to recurrence

The lungs are a significant organ site of murine cytomegalovirus (mCMV) latency. We have shown that activity of the major immediate-early promoter (MIEP), which drives the transcription from the ie1-ie3 transcription unit, does not inevitably initiate the productive cycle (S. K. Kurz, M. Rapp, H.-P. Steffens, N. K. A. Grzimek, S. Schmalz, and M. J. Reddehase, J. Virol. 73:482-494, 1999). Thus, even though MIEP activity governed by the MIEP-enhancer is unquestionably the first condition for recurrence, regulation of the enhancer by transcription factors is not the only mechanism controlling latency. Specifically, during latency, focal and stochastic MIEP activity in lung tissue was found to selectively generate IE1 transcripts, while transactivator-specifying IE3 transcripts were missing. This suggested a control of mCMV latency that is effectual at IE1-IE3 precursor mRNA cotranscriptional processing. Here we have used this model for studying the kinetics of reactivation and recurrence in individual lung tissue pieces after hematoablative, genotoxic treatment. Notably, reactivation was triggered, but the number of transcriptionally active foci in the lungs did not increase over time. This result is not compatible with a model of spontaneous reactivations accumulating after withdrawal of immune control. Instead, the data support the idea that reactivation is an induced event. In some pieces, focal reactivation generated IE3 transcripts but not gB transcripts, while other pieces contained foci that had proceeded to gB transcription, and only a few foci actually reached the state of virus recurrence. This finding indicates the existence of several sequentially ordered control points in the transition from mCMV latency to recurrence.

In vivo replication of recombinant murine cytomegalovirus driven by the paralogous major immediate-early promoter-enhancer of human cytomegalovirus

Transcription of the major immediate-early (MIE) genes of cytomegaloviruses (CMV) is driven by a strong promoter-enhancer (MIEPE) complex. Transactivator proteins encoded by these MIE genes are essential for productive infection. Accordingly, the MIEPE is a crucial control point, and its regulation by activators and repressors is pertinent to virus replication. Since the MIEPE contains multiple regulatory elements, it was reasonable to assume that specific sequence motifs are irreplaceable for specifying the cell-type tropism and replication pattern. Recent work on murine CMV infectivity (A. Angulo, M. Messerle, U. H. Koszinowski, and P. Ghazal, J. Virol. 72:8502-8509, 1998) has documented the proposed enhancing function of the enhancer in that its resection or its replacement by a nonregulatory stuffer sequence resulted in a significant reduction of infectivity, even though replication competence was maintained by a basal activity of the spared authentic MIE promoter. Notably, full capacity for productive in vitro infection of fibroblasts was restored in recombinant viruses by the human CMV enhancer. Using two-color in situ hybridization with MIEPE-specific polynucleotide probes, we demonstrated that a murine CMV recombinant in which the complete murine CMV MIEPE is replaced by the paralogous human CMV core promoter and enhancer (recombinant virus mCMVhMIEPE) retained the potential to replicate in vivo in all tissues relevant to CMV disease. Notably, mCMVhMIEPE was also found to replicate in the liver, a site at which transgenic hCMV MIEPE is silenced. We conclude that productive in vivo infection with murine CMV does not strictly depend on a MIEPE type-specific regulation.

Focal transcriptional activity of murine cytomegalovirus during latency in the lungs

Interstitial pneumonia is a frequent and critical manifestation of human cytomegalovirus (CMV) disease in immunocompromised patients, in particular in recipients of bone marrow transplantation. Previous work in the murine CMV infection model has identified the lungs as a major organ site of CMV latency and recurrence. It was open to question whether the viral genome is transcriptionally silent or active during latency. Transcription could be latency associated and thus be part of the latency phenotype. Alternatively, transcriptional activity could reflect episodes of reactivation. We demonstrate here that transcription of the immediate-early (IE) transcription unit ie1-ie3 selectively generates ie1-specific transcripts during latency. Notably, while the latent viral DNA was found to be evenly distributed in the lungs, transcription was focal and randomly distributed. This finding indicates that IE transcription is not a feature inherent to murine CMV latency but rather reflects foci of primordial reactivation. However, this reactivation did not initiate productive infection, since ie3 gene mRNA specifying the essential transactivator IE3 of murine CMV early gene expression was not detectable. Accordingly, transcripts encoding gB were absent during latency. By contrast, during induced virus recurrence, IE-phase transcription switched from focal to generalized and ie3-specific transcripts were generated. These data imply that latency and recurrence are regulated not only at the IE promoter-enhancer and that there exists an additional checkpoint at the level of precursor RNA splicing. We propose that focal transcription reflects random episodes of nonproductive reactivation that get terminated before IE3 is expressed and ignites the productive cycle.

Control of cytomegalovirus in bone marrow transplantation chimeras lacking the prevailing antigen-presenting molecule in recipient tissues rests primarily on recipient-derived CD8 T cells

Cytomegalovirus (CMV) infection during the transient immunodeficiency after bone marrow transplantation (BMT) develops into disease unless antiviral CD8 T cells are restored in due course. Histoincompatibility between donor and recipient is associated with increased risk. Complications may include a rejection response against the foreign major histocompatibility complex (MHC) antigens and a lack of antiviral control resulting from a misfit between donor-derived T cells and the antigenic viral peptides presented in recipient tissues. Here we have established a murine model of CMV disease after experimental BMT performed across a single MHC class I disparity. Specifically, BALB/c bone marrow cells expressing the prevailing antigen-presenting molecule Ld were transplanted into the Ld gene deletion mutant BALB/c-H-2(dm2), an experimental setting that entails a selective risk of host-versus-graft but not graft-versus-host response. The reconstituted T-cell population proved to be chimeric in that it consisted of Ld-positive donor-derived and Ld-negative recipient-derived cells. Pulmonary infiltrates did not include cytolytic T cells directed against Ld. This finding implies that the infection did not trigger a host-versus-graft response. Notably, upon adoptive transfer, donor-derived CD8 T cells preferentially protected tissues of donor genotype, whereas recipient-derived CD8 T cells protected tissues of either genotype. We infer from these data that the focus on immunodominant antigens presented by Ld within the donor cell population distracted the donor T cells from protecting recipient tissues and that protection in the chimeras was therefore primarily based on recipient T cells. As a consequence, T-cell chimerism after BMT should give a positive prognosis with respect to control of CMV.

Reconstitution of CD8 T cells is essential for the prevention of multiple-organ cytomegalovirus histopathology after bone marrow transplantation

Cytomegalovirus (CMV) infection in the period of temporary immunodeficiency after haematoablative treatment and bone marrow transplantation (BMT) is associated with a risk of graft failure and multiple-organ CMV disease. The efficacy of immune system reconstitution is decisive for the prevention of CMV pathogenesis after BMT. Previous data in murine model systems have documented a redundancy in the immune effector mechanisms controlling CMV. CD8 T cells proved to be relevant but not irreplaceable as antiviral effectors. Specifically, in a state of long-term in vivo depletion of the CD8 T-cell subset, CD4 T cells were educed to become deputy effectors controlling CMV by a mechanism involving antiviral cytokines. It is of medical importance to know whether one can trust in this 'flexible defence' in all clinical settings. It is demonstrated here that reconstitution of CD8 T cells is crucial for the prevention of fatal multiple-organ CMV disease under the specific conditions of BMT.

Control of murine cytomegalovirus in the lungs: relative but not absolute immunodominance of the immediate-early 1 nonapeptide during the antiviral cytolytic T-lymphocyte response in pulmonary infiltrates

The lungs are a major organ site of cytomegalovirus (CMV) infection, pathogenesis, and latency. Interstitial CMV pneumonia represents a critical manifestation of CMV disease, in particular in recipients of bone marrow transplantation (BMT). We have employed a murine model for studying the immune response to CMV in the lungs in the specific scenario of immune reconstitution after syngeneic BMT. Control of pulmonary infection was associated with a vigorous infiltration of the lungs, which was characterized by a preferential recruitment and massive expansion of the CD8 subset of alpha/beta T cells. The infiltrate provided a microenvironment in which the CD8 T cells differentiated into mature effector cells, that is, into functionally active cytolytic T lymphocytes (CTL). This gave us the opportunity for an ex vivo testing of the antigen specificities of CTL present at a relevant organ site of viral pathogenesis. The contribution of the previously identified immediate-early 1 (IE1) nonapeptide of murine CMV was evaluated by comparison with the CD3epsilon-redirected cytolytic activity used as a measure of the overall CTL response in the lungs. The IE1 peptide was detected by pulmonary CTL, but it accounted for a minor part of the response. Interestingly, no additional viral or virus-induced antigenic peptides were detectable among naturally processed peptides derived from infected lungs, even though infected fibroblasts were recognized in a major histocompatibility complex-restricted manner. We conclude that the antiviral pulmonary immune response is a collaborative function that involves many antigenic peptides, among which the IE1 peptide is immunodominant in a relative sense.

Proliferation and MHC-unrestricted bystander lysis by virus-specific cytotoxic T cells following antigen self-presentation

Cytotoxic T cells (CTL) not only act as effector cells, but can also serve as antigen-presenting cells (APC) for other CTL due to their expression of major histocompatibility complex (MHC) class I molecules. In the present study we show that independently derived CTL lines (CTLL) with specificity for an L(d)-presented nonapeptide corresponding to amino acids 168-176 of the immediate-early 1 (IE1) protein of murine cytomegalovirus not only lyse syngeneic but also allogeneic target cells, if the peptide is present during the cytolytic assay. Whereas a short peptide pulse is sufficient to render syngeneic cells susceptible to lysis, continued presence of soluble peptide is mandatory for the lysis of allogeneic target cells. This indicates a difference in the mechanisms involved. Syngeneic BALB/c B cell blasts (K(d)D(d)L(d)) and mutant BALB/c-H-2dm2 B cell blasts lacking the restricting Ld molecules (K(d)D(d)0) were lysed to a similar extent in the absence of the IE1 nonapeptide, provided that the IE1-specific CTL had been pre-incubated with the peptide before the cytolytic assay. Since the mutant cells cannot present the IE1 peptide, their lysis indicates an MHC-unrestricted, peptide-independent mode of recognition by the CTLL. In addition, proliferation of the CTLL takes place after incubation with the cognate peptide, even in the absence of professional APC. These data indicate inter-CTL antigen self-presentation, resulting in activation of the lytic machinery leading to peptide-independent bystander lysis of allogeneic as well as syngeneic target cells.

Cytomegalovirus inhibits the engraftment of donor bone marrow cells by downregulation of hemopoietin gene expression in recipient stroma

Cytomegalovirus (CMV) disease after bone marrow (BM) transplantation is often associated with BM graft failure. There are two possible reasons for such a correlation. First, a poor hematopoietic reconstitution of unrelated etiology could promote the progression of CMV infection by the lack of immune control. Alternatively, CMV infection could interfere with the engraftment of donor BM cells in recipient BM stroma. Evidence for a causative role of CMV in BM aplasia came from studies in long-term BM cultures and from the murine in vivo model of CMV-induced aplastic anemia. A deficiency in the expression of essential stromal hemopoietins, such as stem cell factor (SCF), has indicated a functional insufficiency of the stromal microenvironment. It remained open to question whether CMV mediates a negative regulation of hemopoietin gene expression (the downregulation model) or whether it causes the default of a positive regulator (the lack-of-induction model). Further, even though implicitly assumed, it has never been formally documented that CMV directly interferes with the engraftment of a BM cell transplant. We addressed these problems in a murine model of CMV infection after experimental male-into-female BM transplantation. The data indicate that the downregulation model applies. Quantitation of the male-sex-determining gene tdy demonstrated an impaired engraftment of donor BM cells in the BM stroma of the female recipients. This graft failure was reflected by a diminished population of SCF-receptor-expressing hematopoietic progenitor cells and correlated with a reduced level of stromal SCF gene expression. Interestingly, high doses of BM cells protected against stromal insufficiency by a mechanism unrelated to control of infection.

Evidence against a key role for transforming growth factor-beta1 in cytomegalovirus-induced bone marrow aplasia

During immunodeficiency after sublethal haematoablative treatment, cytomegalovirus (CMV) infection interferes with haematopoietic reconstitution and can cause lethal bone marrow (BM) aplasia. The in vivo model of murine CMV infection has identified the BM stroma as the principal target site of CMV in the haematopoietic cord. The infected cell type is the reticular stromal cell which forms the stromal network and produces essential haemopoietins, such as stem-cell factor (SCF). The expression of SCF was found to be reduced in the infected stroma, but the stromal network was not disrupted and the number of infected stromal cells was too low to explain the functional deficiency. These facts call for a negatively regulating cytokine that is induced by the infection and that potentiates the direct effect of infection by down-regulating haemopoietins in uninfected bystander cells. Recent work has suggested that transforming growth factor (TGF)-beta1 might be the cytokine involved in CMV-induced BM aplasia. We show here that murine CMV indirectly induces the accumulation of mature TGF-beta1 in uninfected renal tubular epithelial cells and TGF-beta1 transcription in BM stromal cells, whereas infected renal glomerular and interstitial cells, hepatocytes and BM stromal cells do not coexpress mature TGF-beta1. Antiviral CD8 T-cell therapy prevented BM aplasia and also prevented the down-regulation of stromal SCF and interleukin-6 gene expression. Interestingly, however, the CD8 T cells did not preclude the up-regulation of mature TGF-beta1, but proved to be inducers of TGF-beta1 gene expression in BM stroma. These findings suggest that TGF-beta1 is not the mediator of BM aplasia.

Preemptive CD8 T-cell immunotherapy of acute cytomegalovirus infection prevents lethal disease, limits the burden of latent viral genomes, and reduces the risk of virus recurrence

In the immunocompetent host, primary cytomegalovirus (CMV) infection is resolved by the immune response without causing overt disease. The viral genome, however, is not cleared but is maintained in a latent state that entails a risk of virus recurrence and consequent organ disease. By using murine CMV as a model, we have shown previously that multiple organs harbor latent CMV and that reactivation occurs with an incidence that is determined by the viral DNA load in the respective organ (M. J. Reddehase, M. Balthesen, M. Rapp, S. Jonjic, I. Pavic, and U. H. Koszinowski. J. Exp. Med. 179:185-193, 1994). This predicts that a therapeutic intervention capable of limiting the load of latent viral genome should also reduce the risk of virus recurrence. Here we demonstrate the benefits and the limits of a preemptive CD8 T-cell immunotherapy of CMV infection in the immunocompromised bone marrow transplantation recipient. Antiviral CD8 T cells prevented CMV disease and accelerated the resolution of productive infection. The therapy also resulted in a lower load of latent CMV DNA in organs and consequently reduced the incidence of recurrence. The data thus provide a further supporting argument for clinical trials of preemptive cytoimmunotherapy of human CMV disease with CD8 T cells. However, CD8 T cells failed to clear the viral DNA. The therapy-susceptible portion of the DNA load differed between organs and was highest in the lungs. The existence of an invariant, therapy-resistant load suggests a role for immune system evasion mechanisms in the establishment of CMV latency.

Bone marrow failure by cytomegalovirus is associated with an in vivo deficiency in the expression of essential stromal hemopoietin genes

Bone marrow (BM) failure associated with cytomegalovirus (CMV) infection is a feared complication after clinical BM transplantation. Experiments in long-term BM cultures have indicated that BM stromal cells (BMSC) are targets of productive CMV infection, but an in situ infection of BM stroma remained to be documented, and the pathomechanism is open to question. Here we describe a murine in vivo model of lethal CMV aplastic anemia (CMV-AA). The reconstitution of hematopoietic progenitor cells expressing stem cell factor (SCF) receptor was found to be defective in CMV-AA. While murine CMV replication in permissive parenchymal tissues is cytolytic, the hematopoietic cord was found to be a site of very limited virus production with foci of reticular BMSC expressing the intranuclear viral IE1 protein, but with only a few BMSC positive for viral genome in the in situ hybridization. XX-XY BM chimeras were established in order to quantitate Y-chromosome-tagged BMSC by a PCR specific for the male-sex-determining gene Tdy. This approach revealed that murine CMV infection is not associated with a significant loss of BMSC. Despite the physical integrity of the stromal network, the functional integrity of the stroma was impaired. While housekeeping genes were expressed normally in BMSC of infected mice, the expression of genes encoding the essential hemopoietins SCF, granulocyte colony-stimulating factor, and interleukin-6 was markedly reduced. In conclusion, the mechanism of BM failure is not a stromal lesion but an insufficient stromal function. These findings explain CMV-AA as a manifestation of multiple hemopoietin deficiency.

Latency versus persistence or intermittent recurrences: evidence for a latent state of murine cytomegalovirus in the lungs

The state of cytomegalovirus (CMV) after the resolution of acute infection is an unsolved problem in CMV research. While the term "latency" is in general use to indicate the maintenance of the viral genome, a formal exclusion of low-level persistent productive infection depends on the sensitivity of the assay for detecting infectious virus. We have improved the method for detecting infectivity by combining centrifugal infection of permissive indicator cells in culture, expansion to an infectious focus, and sensitive detection of immediate-early RNA in the infected cells by reverse transcriptase PCR. A limiting-dilution approach defined the sensitivity of this assay. Infectivity was thereby found to require as few as 2 to 9 virion DNA molecules of murine CMV, whereas the standard measure of infectivity, the PFU, is the equivalent of ca. 500 viral genomes. Since murine CMV forms multicapsid virions in most infected tissues, the genome-to-infectivity ratio is necessarily >1. This assay thus sets a new standard for investigating CMV latency. In mice in which acute infection was resolved, the viral DNA load in the lungs, a known organ site of CMV latency and recurrence, was found to be 1 genome per 40 lung cells, or a total of ca. 1 million genomes. Despite this high load of CMV DNA, infectious virus was not detected with the improved assay, but recurrence was inducible. These data provide evidence against a low-level persistent productive infection and also imply that intermittent spontaneous recurrence is not a frequent event in latently infected lungs.

Porcine T-cell receptors: molecular and biochemical characterization

Two subclasses of CD3 associated T-cell receptors (TcR) have been described so far, consisting of either an alpha and beta chain (TcR alpha beta) or a gamma and delta chain (TcR gamma delta). Of the two subclasses, the TcR alpha beta is the one predominantly expressed on peripheral T lymphocytes of humans and rodents. TcR gamma delta T lymphocytes represent only a minor subset in these species. Among all mammalian species studied so far, swine showed the most diversified composition of the T-lymphocyte population characterized by the expression of CD4 and CD8 differentiation antigens. Besides CD4+CD8- and CD4-CD8+ T lymphocytes, CD4+CD8+ and CD4-CD8- T lymphocytes are prominent in the extrathymic T-lymphocyte compartment. Because of the lack of specific monoclonal antibodies (mAb), to date the porcine TcR can only be characterized with biochemical and molecular biological methods. TcR on porcine peripheral blood T lymphocytes with the phenotype CD4+ and/or CD8+ are characterized as 46-48 kDaR heterodimers which were supposed to represent the porcine TcR alpha beta. Biochemical analyses of the CD4-CD8- T lymphocytes revealed three distinct TcR gamma delta; all are characterized by a 40 kDa delta chain but differed in their gamma chains. One gamma chain with a molecular mass of 38 kDaR is preferentially expressed on CD4-CD8- T lymphocytes derived from peripheral blood; another chain with molecular mass of 37 kDaR is evenly distributed between CD4-CD8- T lymphocytes from blood and lymphoid tissues.(ABSTRACT TRUNCATED AT 250 WORDS)

The establishment of cytomegalovirus latency in organs is not linked to local virus production during primary infection

Recovery from primary cytomegalovirus (CMV) infection is associated with resolution of the productive infection without clearance of the virus genome from affected organs. The presence of latent CMV genome in multiple organs provides the molecular basis for recurrence of CMV within multiple organs, and explains the diversity in the organ manifestations of recrudescent CMV disease during states of immunodeficiency. As a part of a unifying concept of multifocal CMV latency and recurrence, previous work has demonstrated the importance of primary virus replication for the overall load of latent CMV in organs and the risk of recurrence. In the present report, the establishment of CMV latency was studied in a murine model in which the course of primary infection in the immunocompromised host after syngeneic bone marrow transplantation was modulated by a CD8+ T cell immunotherapy. The antiviral CD8+ effector cells limited virus replication in all organs and protected the recipients from lethal CMV disease, but after resolution of the productive infection virus DNA remained. Interestingly, the copy number of latent virus DNA in tissue did not quantitatively reflect the preceding virus production in the respective organ. Specifically, in contrast to the case in the lungs and the salivary glands, virus replication in the spleen was suppressed by CD8+ T cells to below the limit of detection; yet, virus DNA was also detected in the spleen during latency and accordingly, virus recurrence in the spleen could be induced. These findings demonstrate that the control of virus replication in a particular organ does not prevent the establishment of latency in that organ.

The conditions of primary infection define the load of latent viral genome in organs and the risk of recurrent cytomegalovirus disease

Recurrence of cytomegalovirus (CMV) from latency is a frequent cause of disease in immunocompromised patients. To date, there is no explanation for the diversity in the clinical manifestations. Primary infection can occur perinatally or later in life, and inevitably results in latent infection. Seropositivity for antibodies against CMV is indicative of latent infection, but is insufficient as a predictor for the risk of recurrence. As a model for this important medical problem, we compared the risks of murine CMV recurrence from latency established after neonatal primary infection and after infection at adult age. The risk of CMV recurrence was high only after neonatal infection. The copy number of latent viral genome in tissues was identified as the key parameter that determines the overall and organ-specific risks of recurrence. Latent CMV burden and risk of recurrence were related to the extent of virus multiplication during primary infection. The presence of latent CMV in multiple organs provides the molecular basis for stochastic events of recurrence in single organs or in any combination thereof. These findings are discussed as a concept of multifocal CMV latency and recurrence. It provides a rationale for the diversity in the clinical outcome of CMV disease.

Lungs are a major organ site of cytomegalovirus latency and recurrence

Recurrence of infectious virus from the latent viral genomes is the initiating event in the pathogenesis of cytomegalovirus (CMV) disease during states of immunodeficiency. Interstitial pneumonia is a frequent manifestation of posttransplantation CMV disease, in particular after bone marrow transplantation and heart and lung transplantations. Recurrence can occur within the transplant derived from a latent infected donor as well as within latently infected organs of the transplant recipient. The reason for a predilection of the lungs as a site of CMV pathology is so far unknown. In a murine model of CMV latency, the lungs were identified as an authentic site of latent infection, since the viral genome remained detectable in lung tissue even after it was cleared to an undetectable level in blood and bone marrow. A comparison between the lungs and the spleen, the previously most thoroughly investigated site of murine CMV latency, revealed a 10-fold-higher burden of latent viral genome for the lungs. Most important, the organ-specific risk of in vivo recurrence was found to correlate with the organ-specific viral genomic load. This new finding thus characterizes the lungs as a high-risk organ for CMV recurrence, and this fact may explain in part why interstitial pneumonia is a frequent manifestation of recurrent CMV infection.

Expression of gamma/delta T cell receptors in porcine thymus

Swine possess an extraordinarily high number of T lymphocytes with the phenotype CD4-CD8- in peripheral blood as well as in lymphoid tissues. This subpopulation is subdivided into at least four subsets defined by the expression of CD2 and three biochemically distinct gamma/delta T cell receptors. The four subsets differ largely in their pattern of lymphoid homing in that CD2- subsets, historically referred to as null lymphocytes, predominate in the circulating pool, whereas CD2+ subsets are enriched in lymphoid tissues. Here we document the expression of all three types of gamma/delta T cell receptors by CD4-CD8- porcine thymocytes, which provides the first evidence for a thymic origination of all subsets of porcine gamma/delta T lymphocytes. The biochemical analysis shows that three distinct gamma-chains form disulfide-bonded cell surface heterodimers with a common delta-chain and that glycosylation of all chains is already completed within the thymus. Surprisingly, CD2- subsets, which are known to be enriched among thymic emigrants and which numerically predominate in peripheral blood, are underrepresented in the thymus, suggesting a high export rate.

Rabies virus neuritic paralysis: immunopathogenesis of nonfatal paralytic rabies

Two pathogenetically distinct disease manifestations are distinguished in a murine model of primary rabies virus infection with the Evelyn-Rokitnicky-Abelseth strain, rabies virus neuritic paralysis (RVNP) and fatal encephalopathogenic rabies. RVNP develops with high incidence in immunocompetent mice after intraplantar infection as a flaccid paralysis restricted to the infected limb. The histopathologic correlate of this monoplegia is a degeneration of the myelinated motor neurons of the peripheral nerve involved. While, in this model, fatal encephalopathogenic rabies develops only after depletion of the CD4 subset of T lymphocytes and without contribution of the CD8 subset, RVNP is identified as an immunopathological process in which both the CD4 and CD8 subsets of T lymphocytes are critically implicated.

Hematopoietic stem cell deficiency resulting from cytomegalovirus infection of bone marrow stroma

Cytomegalovirus (CMV) recurrence from latency is a major risk factor in bone marrow transplantation (BMT). Owing to the immunodepletive treatment, ablation of the immune control of latent CMV is responsible for recurrence and cytopathogenic spread of the virus in vital tissues. There is increasing evidence for reconstituting bone marrow being itself a target tissue of CMV. By inhibiting post-transplantation hematopoiesis, CMV is causal for maintenance of the immunocompromised state, which leads to a prolonged phase of persistent virus replication. Based on results in a murine model of BMT and concurrent CMV infection, we discuss possible mechanisms of CMV-mediated bone marrow graft failure. It is concluded that an irremediable damage of bone marrow stroma by CMV is responsible for a reduced rate of regeneration of the marrow-repopulating, pluripotent stem cell.

Molecular modeling of an antigenic complex between a viral peptide and a class I major histocompatibility glycoprotein

Computer simulation of the conformations of short antigenic peptides (5-10 residues) either free or bound to their receptor, the major histocompatibility complex (MHC)-encoded glycoprotein H-2 Ld, was employed to explain experimentally determined differences in the antigenic activities within a set of related peptides. Starting for each sequence from the most probable conformations disclosed by a pattern-recognition technique, several energy-minimized structures were subjected to molecular dynamics simulations (MD) either in vacuo or solvated by water molecules. Notably, antigenic potencies were found to correlate to the peptides propensity to form and maintain an overall alpha-helical conformation through regular i,i + 4 hydrogen bonds. Accordingly, less active or inactive peptides showed a strong tendency to form i,i + 3 hydrogen bonds at their N-terminal end. Experimental data documented that the C-terminal residue is critical for interaction of the peptide with H-2 Ld. This finding could be satisfactorily explained by a 3-D Q.S.A.R. analysis postulating interactions between ligand and receptor by hydrophobic forces. A 3-D model is proposed for the complex between a high-affinity nonapeptide and the H-2 Ld receptor. First, the H-2 Ld molecule was built from X-ray coordinates of two homologous proteins: HLA-A2 and HLA-Aw68, energy-minimized and studied by MD simulations. With HLA-A2 as template, the only realistic simulation was achieved for a solvated model with minor deviations of the MD mean structure from the X-ray conformation. Water simulation of the H-2 Ld protein in complex with the antigenic nonapeptide was then achieved with the template-derived optimal parameters. The bound peptide retains mainly its alpha-helical conformation and binds to hydrophobic residues of H-2 Ld that correspond to highly polymorphic positions of MHC proteins. The orientation of the nonapeptide in the binding cleft is in accordance with the experimentally determined distribution of its MHC receptor-binding residues (agretope residues). Thus, computer simulation was successfully employed to explain functional data and predicts alpha-helical conformation for the bound peptide.

Principles of cytomegalovirus antigen presentation in vitro and in vivo

Cytomegaloviruses are members of the ubiquitous family of herpesviruses. They escape immunological clearance and persist throughout life in the infected host. Yet, the stability of the balance of this virus-host interaction is dependent upon the state of the cellular immune response, and usually requires the function of specific CD8 T lymphocytes. In a mouse model, the major antigen that triggers protective CD8 T lymphocytes has been characterized as a nonameric amino acid motif of a nonstructural virus protein. Analysis of the naturally presented peptide has led to the conclusion that the sequence of an antigenic epitope is not the only parameter that decides upon its efficient presentation. Furthermore, the virus has developed regulatory functions that can prevent antigen presentation in productively infected cells. Contradictions between in vivo and in vitro findings are resolved in a hypothesis postulating an essential contribution of cytokines to the in vivo control of productive cytomegalovirus infection.

Pathogenesis of classical swine fever: B-lymphocyte deficiency caused by hog cholera virus

Hog cholera, also known as classical or European swine fever, is caused by hog cholera virus, a member of the genus Pestivirus. It is shown here that the end stage of lethal infection in the natural host is associated with a dramatic depletion preferentially of B lymphocytes in the circulatory system as well as in lymphoid tissues. Already at the onset of disease, viral replication in lymphoid tissues demarcates the germinal centers, and the viral genome remains localized to that site as the disease progresses even after morphologic disintegration of the follicular structure. A block in B-lymphocyte maturation by infection and destruction of germinal centers is discussed as a key event in the pathogenesis of acute, lethal hog cholera.

Efficient processing of an antigenic sequence for presentation by MHC class I molecules depends on its neighboring residues in the protein

Processing of endogenously synthesized proteins generates short peptides that are presented by MHC class I molecules to CD8 T lymphocytes. Here it is documented that not only the sequence of the presented peptide but also the residues by which it is flanked in the protein determine the efficiency of processing and presentation. This became evident when a viral sequence of proven antigenicity was inserted at different positions into an unrelated carrier protein. Not different peptides, but different amounts of the antigenic insert itself were retrieved by isolation of naturally processed peptides from cells expressing the different chimeric proteins. Low yield of antigenic peptide from an unfavorable integration site could be overcome by flanking the insert with oligo-alanine to space it from disruptive neighboring sequences. Notably, the degree of protection against lethal virus disease related directly to the amount of naturally processed antigenic peptide.

Resting porcine T lymphocytes expressing class II major histocompatibility antigen

The immune system of swine is unique in that the expression of CD4 and CD8 antigens defines four subpopulations of resting, extrathymic (CD1-) T lymphocytes in the circulation as well as in lymphoid tissue. Here it is documented that the specialty of the porcine T lymphocyte compartment extends to the expression of class II MHC (SLA) antigens. While the TCR gamma/delta CD4-CD8- as well as the TCR alpha/beta CD4+CD8- subpopulation both lack MHC II, the TCR alpha/beta CD4-CD8+ and the CD4+CD8+ subpopulation, the latter of which is private to swine, both do express MHC II. As opposed to human T lymphocytes, expression of porcine MHC II is not transient and restricted to lymphoblasts but is imminent in small, resting T lymphocytes of the two CD8-expressing subsets, even though also in swine activation can induce MHC II. Activation-induced extrathymic acquisition of MHC II without reversal can be discussed as one possible way of how MHC II+ T lymphocytes are generated. Alternatively, MHC II antigens should already be expressed by thymic progenitors. Remarkably, all CD1+CD4-CD8+ and most CD1+CD4+CD8+ thymocytes lack MHC II, yet, a minor subset with the phenotype CD4hiCD8hi expresses MHC II. One may speculate that these cells do not undergo thymic selection and represent the progenitors of the unusual, swine-typic MHC II+CD4+CD8+ extrathymic T lymphocytes.

The role of CD4 and CD8 T cells in viral infections

Presentation of viral antigens to T cells does not require uptake by 'professional' antigen-presenting cells. Viruses have specialized to enter the cells in which they replicate. Virus entry, uncoating and new viral protein synthesis can load both the cytosolic and the endosomal pathway of antigen processing, resulting in viral peptide presentation to CD8 and CD4 T cells by MHC class I and II molecules, respectively. Although a role of CD8 T cells in the control of viral infection has been well documented, current research interest centers on the contribution of the different CD4 T-cell subsets.

Redistribution of critical major histocompatibility complex and T cell receptor-binding functions of residues in an antigenic sequence after biterminal substitution

Residues critical for establishing a trimolecular interaction with a major histocompatibility complex (MHC)-encoded receptor and a T cell antigen receptor (TcR) were determined for an antigenic nonapeptide. The N-terminal residue proved to be involved in binding of the peptide to both receptors and the C-terminal residue was essential for MHC binding. While substitution of either of these critical terminal residues by alanine resulted in an almost complete loss of peptide antigenicity, simultaneous substitution of both created a new functional ligand for the same MHC molecule and the same TcR. Notably, in the biterminally substituted peptide, the core residues took on new roles in the trimolecular interaction in that a residue critical in the authentic nonapeptide for TcR binding became critical for MHC binding and former spacer residues became essential to various degrees for the interaction with either receptor or both. Thus, apparently, the loss of the terminal residues' contribution was at least partially compensated by a redistribution of the roles among the remaining residues. These results reflect a cooperative contribution of all residues of an antigenic peptide to its binding to both receptors and thus challenge a static definition of agretope and epitope as MHC and TcR binding sites.

Protection against lethal cytomegalovirus infection by a recombinant vaccine containing a single nonameric T-cell epitope

The regulatory immediate-early (IE) protein pp89 of murine cytomegalovirus induces CD8+ T lymphocytes that protect against lethal murine cytomegalovirus infection. The IE1 epitope is the only epitope of pp89 that is recognized by BALB/c cytolytic T lymphocytes (CTL). Using synthetic peptides, the optimal and minimal antigenic sequences of the IE1 epitope have been defined. To evaluate the predictive value of data obtained with synthetic peptides, recombinant vaccines encoding this single T-cell epitope were constructed using as a vector the hepatitis B virus core antigen encoded in recombinant vaccinia virus. In infected cells expressing the chimeric proteins, only IE1 epitope sequences that were recognized as synthetic peptides at concentrations lower than 10(-6) M were presented to CTL. Vaccination of mice with the recombinant vaccinia virus that encoded a chimeric protein carrying the optimal 9-amino-acid IE1 epitope sequence elicited CD8+ T lymphocytes with antiviral activity and, furthermore, protected against lethal disease. The results thus show for the first time that recombinant vaccines containing a single foreign nonameric CTL epitope can induce T-lymphocyte-mediated protective immunity.

Rescue of myeloid lineage-committed preprogenitor cells from cytomegalovirus-infected bone marrow stroma

The effect of murine cytomegalovirus on myelopoiesis was studied in long-term bone marrow culture to find an in vitro correlate for the lethal virus interference with bone marrow reconstitution (W. Mutter, M. J. Reddehase, F. W. Busch, H.-J. Bühring, and U. H. Koszinowski, J. Exp. Med. 167:1645-1658, 1988). The in vitro generation of granulocyte-monocyte progenitors (CFU-GM) discontinued after infection of the stromal cell layer, whereas the proliferation and differentiation of CFU-GM to granulocyte-monocyte colonies remained unaffected. A protocol was established to probe the functional integrity of earlier hematopoietic cells. Pre-CFU-GM (the progenitors of the CFU-GM) could be recovered from an infected bone marrow donor culture by transfer onto an inductive recipient stromal cell layer. Thus, at least in vitro, infection of bone marrow stroma appears to be the only cause of the defect in myelopoiesis.

Porcine gamma/delta T lymphocyte subsets differing in their propensity to home to lymphoid tissue

Three types of T cell receptors (TcR) gamma/delta were identified in swine, all of which are disulfide-bonded cell surface heterodimers. The first, a 38/40-kDa molecule, is related to the TcR gamma/delta of sheep and cattle, and defines in swine a subset of CD4-CD8- T lymphocytes that is enriched in the circulating T lymphocyte pool. The other two receptors are private to swine. The second, a 37/40-kDa molecule, is expressed by the major subset of CD2-CD4-CD8- T lymphocytes that is neither enriched in nor excluded from lymphoid tissue. By contrast, the third type of TcR gamma/delta, a 40/46-kDa molecule, is found on CD2+CD4-CD8- T lymphocytes accumulating in lymphoid tissue. Thus, TcR gamma/delta T lymphocyte subsets differing in homing propensity are typified by the expression of distinct molecular forms of the TcR gamma/delta.

Distinct gamma/delta T cell receptors define two subsets of circulating porcine CD2-CD4-CD8- T lymphocytes

The species swine provides the only example for CD2+ and CD2- subsets of Ig-CD4-CD8- lymphocytes with the propensity for homing to lymphoid tissue (Saalmüller et al., Eur. J. Immunol. 1989. 19: 2011). That the CD2-CD4-CD8- lymphocytes are bare of marker molecules that typify T lymphocytes raised the question of whether or not this cell type is descended from the T lymphocyte differentiation lineage. It is documented that expression of a phylogenetically conserved external epitope of T cell receptor gamma/delta subdivides porcine CD2- lymphocytes into an epitope 86D+ minor and an 86D- major subset. Expression of distinct forms of the T cell receptor gamma/delta, disulfide-bonded N-glycosylated surface heterodimers of under reducing conditions 38/40 and 37/40 kDa, respectively, hallmarks the CD2-86D+ and CD2-86D- subsets both as T lymphocytes.

Phenotypic discrimination between thymic and extrathymic CD4-CD8- and CD4+CD8+ porcine T lymphocytes

The composition of the T lymphocyte population in swine is special in that in addition to the CD4-CD8+ subpopulations, CD4-CD8- and CD4+CD8- and CD4+CD8+ subpopulations are prominent in the peripheral circulating as well as in the resident T lymphocyte pools. Since the same phenotypes are characteristic of thymic populations, it was asked whether the unusual distribution in swine may result from an emigration of thymic precursor phenotypes to the periphery. This explanation was refuted, as all thymic subpopulations were found to express CD1, albeit with differences in antigen density, whereas all extrathymic subpopulations lack CD1. The cellular distribution of CD2 in swine is without precedent among all species studied. Whereas in sheep and cattle the extrathymic CD4-CD8- subpopulation is known to entirely lack CD2 and to have a low propensity for homing to lymphoid tissues, the CD4-CD8- subpopulation in swine splits into CD2+ and CD2- subsets, both of which do reside in lymphoid tissues. While CD2+CD4-CD8- T lymphocytes are rare in the circulating pool, this subset accumulates in spleen and lymph nodes. This may indicate a role for CD2 in homing. Thus the species swine is immunologically unique, not only because of having CD1-CD2+CD4+CD8+ T lymphocytes in the periphery, but also with regard to subdivision and homing behavior of its CD4-CD8- T lymphocyte subpopulation.

Structure of the gene of tum- transplantation antigen P91A: the mutated exon encodes a peptide recognized with Ld by cytolytic T cells

Mutagen treatment of mouse P815 tumor cells produces immunogenic mutants that express new transplantation antigens (tum- antigens) recognized by cytolytic T cells. We found that the gene conferring expression of tum- antigen P91A contains 12 exons, encoding a 60 kd protein lacking a typical N-terminal signal sequence. The sequence shows no significant similarity with sequences in current data bases. A mutation that causes expression of the antigen is located in exon 4; it is the only apparent difference between the normal and the antigenic alleles. A short synthetic peptide corresponding to a region of exon 4 located around this mutation makes P815 cells sensitive to lysis by anti-P91A cytolytic T cells. The mutation creates a strong aggretope enabling the peptide to bind the H-2 Ld molecule. Several secondary tumor cell variants that no longer express tum- antigen P91A were found to carry deletions in the gene.

Presentation of CMV immediate-early antigen to cytolytic T lymphocytes is selectively prevented by viral genes expressed in the early phase

The regulation of antigen processing and presentation to MHC class I-restricted cytolytic T lymphocytes was studied in cells infected with murine cytomegalovirus. Recognition by cytolytic T lymphocytes of the phosphoprotein pp89, the immunodominant viral antigen expressed in the immediate-early phase of infection, was selectively prevented during the subsequent expression of viral early genes. The surface expression of MHC class I glycoproteins and their capacity to present externally added pp89-derived antigenic peptides were not affected. Because recognition of several other antigens occurred during the early phase, a general failure in processing and presentation was excluded. Since neither rate of synthesis, amount, stability, nor nuclear transport of pp89 was modified, the failure in recognition indicates a selective interference with pp89 antigen processing and presentation.

Site-restricted persistent cytomegalovirus infection after selective long-term depletion of CD4+ T lymphocytes

We have established a murine model system for exploring the ability of a CD4 subset-deficient host to cope with cytomegalovirus infection, and reported three findings. First, an antiviral response of the CD8 subset of T lymphocytes could be not only initiated but also maintained for a long period of time despite a continued absence of the CD4 subset, whereas the production of antiviral antibody proved strictly dependent upon help provided by the CD4 subset. Second, no function in the defense against infection could be ascribed as yet to CD4-CD8- T lymphocytes, which were seen to accumulate to a new subset as a result of depletion of the CD4 subset. This newly arising subset did not substitute for CD4+ T lymphocytes in providing help to B lymphocytes, and was also not effective in controlling the spread of virus in host tissues. As long as a function of these cells in the generation and maintenance of a CD8 subset-mediated response is not disproved, caution is indicated with concern to an autonomy of the CD8 subset. Third, even though with delay, the CD8+ effector cells raised in the CD4 subset-deficient host were able of clear vital tissues from productive infection and to restrict asymptomatic, persistent infection to acinar glandular epithelial cells in salivary gland tissue.

A pentapeptide as minimal antigenic determinant for MHC class I-restricted T lymphocytes

Peptides that are antigenic for T lymphocytes are ligands for two receptors, the class I or II glycoproteins that are encoded by genes in the major histocompatibility complex, and the idiotypic alpha/beta chain T-cell antigen receptor. That a peptide must bind to an MHC molecule to interact with a T-cell antigen receptor is the molecular basis of the MHC restriction of antigen-recognition by T lymphocytes. In such a trimolecular interaction the amino-acid sequence of the peptide must specify the contact with both receptors: agretope residues bind to the MHC receptor and epitope residues bind to the T-cell antigen receptor. From a compilation of known antigenic peptides, two algorithms have been proposed to predict antigenic sites in proteins. One algorithm uses linear motifs in the sequence, whereas the other considers peptide conformation and predicts antigenicity for amphipathic alpha-helices. We report here that a systematic delimitation of an antigenic site precisely identifies a predicted pentapeptide motif as the minimal antigenic determinant presented by a class I MHC molecule and recognized by a cytolytic T lymphocyte clone.