Natural killer gene complex (Nkc) allelic variability in inbred mice: evidence for Nkc haplotypes

Allelic variability for mouse Chromosome 6 Nkc loci was assessed in 22 common laboratory strains of mice using selected natural killer gene complex (Nkc)-linked sequence tagged site markers. Most Nkc markers distinguished three or more alleles for a particular locus in the assessed mouse strains. Nkc locus alleles were highly conserved among genealogically related inbred strains, whereas far less similarity was observed among unrelated strains. Concurrent strain-to-strain comparisons for all Nkc-linked loci revealed common and uncommon Nkc haplotypes, including some that were likely recombinant. Nkc allele and haplotype assignments in inbred mouse strains and correlation with phenotypic traits should facilitate positional gene cloning strategies for unknown Nkc-linked trait modification loci.

CD1d-restricted NKT cells: an interstrain comparison

CD1d-restricted Valpha14-Jalpha281 invariant alphabetaTCR(+) (NKT) cells are well defined in the C57BL/6 mouse strain, but they remain poorly characterized in non-NK1.1-expressing strains. Surrogate markers for NKT cells such as alphabetaTCR(+)CD4(-)CD8(-) and DX5(+)CD3(+) have been used in many studies, although their effectiveness in defining this lineage remains to be verified. Here, we compare NKT cells among C57BL/6, NK1.1-congenic BALB/c, and NK1.1-congenic nonobese diabetic mice. NKT cells were identified and compared using a range of approaches: NK1.1 expression, surrogate phenotypes used in previous studies, labeling with CD1d/alpha-galactosylceramide tetramers, and cytokine production. Our results demonstrate that NKT cells and their CD4/CD8-defined subsets are present in all three strains, and confirm that nonobese diabetic mice have a numerical and functional deficiency in these cells. We also highlight the hazards of using surrogate phenotypes, none of which accurately identify NKT cells, and one in particular (DX5(+)CD3(+)) actually excludes these cells. Finally, our results support the concept that NK1.1 expression may not be an ideal marker for CD1d-restricted NKT cells, many of which are NK1.1-negative, especially within the CD4(+) subset and particularly in NK1.1-congenic BALB/c mice.

Vital involvement of a natural killer cell activation receptor in resistance to viral infection

Natural killer (NK) cells are lymphocytes that can be distinguished from T and B cells through their involvement in innate immunity and their lack of rearranged antigen receptors. Although NK cells and their receptors were initially characterized in terms of tumor killing in vitro, we have determined that the NK cell activation receptor, Ly-49H, is critically involved in resistance to murine cytomegalovirus in vivo. Ly-49H requires an immunoreceptor tyrosine-based activation motif (ITAM)-containing transmembrane molecule for expression and signal transduction. Thus, NK cells use receptors functionally resembling ITAM-coupled T and B cell antigen receptors to provide vital innate host defense.

NK1.1+ cells and murine cytomegalovirus infection: what happens in situ?

NK cells mediate early host defense against viral infection. In murine CMV (MCMV) infection NK cells play a critical role in controlling viral replication in target organs, such as spleen and liver. Until now it has not been possible to directly examine the role of NK cells in MCMV-induced inflammation in situ due to the inability to stain specifically for NK cells in infected tissues. In this study, we describe a method of in vivo fixation, resulting in the first identification of NK cells in situ using NK1.1 as the marker. Using this method, we characterize the NK1.1(+) cellular component of the inflammatory response to wild-type MCMV in the spleen, liver, and lung of genetically susceptible and resistant mice following i.p. infection. This study provides the first in situ description of the cellular response mediated specifically by NK cells following MCMV infection.

Why do cultured transplanted myoblasts die in vivo? DNA quantification shows enhanced survival of donor male myoblasts in host mice depleted of CD4+ and CD8+ cells or Nk1.1+ cells

Overcoming the massive and rapid death of injected donor myoblasts is the primary hurdle for successful myoblast transfer therapy (MTT), designed as a treatment for the lethal childhood myopathy Duchenne muscular dystrophy. The injection of male myoblasts into female host mice and quantification of surviving male DNA using the Y-chromosome-specific (Y1) probe allows the speed and extent of death of donor myoblasts to be determined. Cultured normal C57BL/10Sn male donor myoblasts were injected into untreated normal C57BL/10Sn and dystrophic mdx female host mice and analyzed by slot blots using a 32P-labeled Y1 probe. The amount of male DNA from donor myoblasts showed a remarkable decrease within minutes and by 1 h represented only about 10-18% of the 2.5 x 10(5) cells originally injected (designated 100%). This declined further over 1 week to approximately 1-4%. The host environment (normal or dystrophic) as well as the extent of passaging in tissue culture (early "P3" or late "P15-20" passage) made no difference to this result. Modulation of the host response by CD4+/CD8+ -depleting antibodies administered prior to injection of the cultured myoblasts dramatically enhanced donor myoblast survival in dystrophic mdx hosts (15-fold relative to untreated hosts after 1 week). NK1.1 depletion also dramatically enhanced donor myoblast survival in dystrophic mdx hosts (21-fold after 1 week) compared to untreated hosts. These results provide a strategic approach to enhance donor myoblast survival in clinical trials of MTT.

Localization on a physical map of the NKC-linked Cmv1 locus between Ly49b and the Prp gene cluster on mouse chromosome 6

The Cmv1 locus controls NK cell-mediated resistance to infection with murine CMV. Our recent genetic analysis of backcross mice demonstrated that the NK gene complex (NKC)-linked Cmv1 locus should reside between the Ly49 and Prp gene clusters on distal mouse chromosome 6. We have aligned yeast artificial chromosome (YAC) inserts in a contig spanning the interval between the Ly49 and Prp gene clusters. This YAC contig includes 13 overlapping YAC inserts that span more than 2 megabases (Mb) in C57BL/6 (B6) mice. Since we have identified genomic clones that span the Ly49-Prp gene region, we hypothesize that at least one should contain the Cmv1 locus. To narrow the Cmv1 critical region, we developed novel NKC genetic markers and used these to genotype informative backcross and intra-NKC recombinant congenic mouse DNA samples. These data suggest that Cmv1 resides on a single YAC insert within an interval that corresponds to a physical distance of approximately 390 kb. This high resolution, integrated physical and genetic NKC map will facilitate identification of Cmv1 and other NKC-linked loci that regulate NK cell-mediated immunity.

Perforin is a major contributor to NK cell control of tumor metastasis

We provide the first demonstration, using experimental and spontaneous models of metastasis in C57BL/6 (B6) (RM-1 prostate carcinoma) and BALB/c (DA3 mammary carcinoma) mice, that tumor metastasis is primarily controlled by perforin-dependent cytotoxicity mediated by NK1.1+ cells. MHC class Ilow RM-1 and DA3 tumor cells were sensitive in vitro to Fas-mediated lysis or spleen NK cells in a perforin-dependent fashion. Perforin-deficient NK cells did not lyse these tumors, and perforin-deficient mice were 10-100-fold less proficient than wild-type mice in rejecting the metastasis of tumor cells to the lung. Fas ligand mutant gld mice displayed uncompromised protection against tumor metastasis. Depletion of NK subsets resulted in greater numbers of metastases than observed in perforin-deficient mice, suggesting that perforin-independent effector functions of NK cells may also contribute to protection from tumor metastasis.

The natural killer gene complex genetic locus Chok encodes Ly-49D, a target recognition receptor that activates natural killing

Previously, we established that natural killer (NK) cells from C57BL/6 (B6), but not BALB/c, mice lysed Chinese hamster ovary (CHO) cells, and we mapped the locus that determines this differential CHO-killing capacity to the NK gene complex on chromosome 6. The localization of Chok in the NK gene complex suggested that it may encode either an activating or an inhibitory receptor. Here, results from a lectin-facilitated lysis assay predicted that Chok is an activating B6 NK receptor. Therefore, we immunized BALB/c mice with NK cells from BALB.B6-Cmv1(r) congenic mice and generated a mAb, designated 4E4, that blocked B6-mediated CHO lysis. mAb 4E4 also redirected lysis of Daudi targets, indicating its reactivity with an activating NK cell receptor. Furthermore, only the 4E4(+) B6 NK cell subset mediated CHO killing, and this lysis was abrogated by preincubation with mAb 4E4. Flow cytometric analysis indicated that mAb 4E4 specifically reacts with Ly-49D but not Ly-49A, B, C, E, G, H, or I transfectants. Finally, gene transfer of Ly-49DB6 into BALB/c NK cells conferred cytotoxic capacity against CHO cells, thus establishing that the Ly-49D receptor is sufficient to activate NK cells to lyse this target. Hence, Ly-49D is the Chok gene product and is a mouse NK cell receptor capable of directly triggering natural killing.

Peptide based cytotoxic T-cell vaccines; delivery of multiple epitopes, help, memory and problems

Synthetic CD8+ cytotoxic T-lymphocyte (CTL) peptide epitope based vaccines are being developed against a number of human diseases. Here we describe extensive preclinical testing of peptide epitope vaccines formulated with a protein as a source of CD4 help and Montanide ISA 720, an adjuvant currently in human clinical trials. Such water-in-oil formulations could effectively co-deliver several peptide epitopes and simultaneously induce multiple independent CTL responses. The efficiency of CTL induction by some peptides was, however, dependent on the aqueous buffer conditions, with poor performance correlating with non-covalent peptide oligomerisation. Any of a number of proteins currently used in human vaccines could supply CD4 help and no difference in CTL induction was obtained if the CD4 response was amnestic or a primary. Peptide immunisation was found to induce long term CTL memory and the recall of protective responses did not depend on an amnestic CD4 response. Slow pyroglutamic acid formation and rapid oxidation of methionine residues was observed in water-in-oil formulations, however, the latter had no effect on CTL induction. These data highlight the need to monitor for potential deleterious chemical events and interpeptide interactions, but illustrate that peptide based vaccination can effectively deliver multiple epitopes, in conjunction with any protein, and induce protective memory.

Genetic control of natural killing and in vivo tumor elimination by the Chok locus

The molecular mechanisms underlying target recognition during natural killing are not well understood. One approach to dissect the complexities of natural killer (NK) cell recognition is through exploitation of genetic differences among inbred mouse strains. In this study, we determined that interleukin 2-activated BALB/c-derived NK cells could not lyse Chinese hamster ovary (CHO) cells as efficiently as C57BL/6-derived NK cells, despite equivalent capacity to kill other targets. This strain-determined difference was also exhibited by freshly isolated NK cells, and was determined to be independent of host major histocompatibility haplotype. Furthermore, CHO killing did not correlate with expression of NK1.1 or 2B4 activation molecules. Genetic mapping studies revealed linkage between the locus influencing CHO killing, termed Chok, and loci encoded within the NK gene complex (NKC), suggesting that Chok encodes an NK cell receptor specific for CHO cells. In vivo assays recapitulated the in vitro data, and both studies determined that Chok regulates an NK perforin-dependent cytotoxic process. These results may have implications for the role of NK cells in xenograft rejection. Our genetic analysis suggests Chok is a single locus that affects NK cell-mediated cytotoxicity similar to other NKC loci that also regulate the complex activity of NK cells.

Murine Nkg2d and Cd94 are clustered within the natural killer complex and are expressed independently in natural killer cells

Natural killer (NK) cells express C-type lectin-like receptors, encoded in the NK gene complex, that interact with major histocompatibility complex class I and either inhibit or activate functional activity. Human NK cells express heterodimers consisting of CD94 and NKG2 family molecules, whereas murine NK cells express homodimers belonging to the Ly-49 family. The corresponding orthologues for other species, however, have not been described. In this report, we used probes derived from the expressed sequence tag database to clone C57BL/6-derived cDNAs homologous to human NKG2-D and CD94. Among normal tissues, murine NKG2-D and CD94 transcripts are highly expressed only in activated NK cells, including both Ly-49A+ and Ly-49A- subpopulations. Additionally, mNKG2-D is expressed in murine NK cell clones KY-1 and KY-2, whereas mCD94 expression is observed only in KY-1 cells but not KY-2. Last, we have finely mapped the physical location of the Cd94 (centromeric) and Nkg2d (telomeric) genes between Cd69 and the Ly49 cluster in the NK complex. Thus, these data indicate the expanding complexity of the NK complex and the corresponding repertoire of C-type lectin-like receptors on murine NK cells.

The murine cytomegalovirus (MCMV) homolog of the HCMV phosphotransferase (UL97(pk)) gene

The murine cytomegalovirus (MCMV) M97 gene is homologous with both eukaryotic protein kinases and the phosphotransferases of herpesviruses. The gene conserves the domain structure of protein kinases and of the human cytomegalovirus UL97 (phosphotransferase) gene. An M97 transcript of 2.5 kb is present predominantly at late times, and much smaller quantities of the transcript are detected at early times postinfection. Comparison of the DNA sequences of the complete M97 genes from 12 ganciclovir-sensitive and aciclovir-sensitive strains of MCMV showed that the sensitive isolates strongly conserve the sequence of the catalytic domains, but have only moderate conservation of the sequence of the amino-terminal (regulatory) region. MCMV provides a useful model for studying the in vivo function of the phosphotransferase genes of the betatherpesviruses and has potential for use in studies of antiviral resistance.

A 2-Mb YAC contig and physical map of the natural killer gene complex on mouse chromosome 6

We have constructed a physical map of a > 2-Mb region on mouse chromosome 6 that contains the natural killer gene complex (NKC). The map comprises a contig of 14 overlapping yeast artificial chromosomes onto which we positioned 25 NKC markers. NKC genetically linked genes encode > 17 proteins that directly control innate NK cell-mediated tumor lysis and disease resistance. Herein we show that Nkrp1 genes are clustered in a region flanked by A2m and Cd69 genes and that most Ly49 genes are clustered in a distal region -1 Mb distant. Importantly, syntenic intervals of mouse chromosome 6 and human chromosome 12p that include the NKC are conserved. NKC species conservation suggests that the human NKC may contain orthologues for the mouse viral disease resistance genes, Cmv1 and Rmp1. The high-resolution NKC map will facilitate investigation of NKC gene regulation and identification of phenotypically defined gene products that confer NK cell defense against viral pathogens.

The Cmv1 host resistance locus is closely linked to the Ly49 multigene family within the natural killer cell gene complex on mouse chromosome 6

Natural killer (NK) cells play important roles in controlling tumor cells and against a range of infectious organisms. Recent studies of mouse NK cell surface receptors, which may be involved in the specificity of NK cells, have shown that many of these molecules are encoded by the Ly49 and Ly55 (Nkrp1) multigene families that map to distal mouse chromosome 6. Also mapping to this NK cell gene complex (NKC) is the resistance locus, Cmv1, which is involved in genetically determined resistance to murine cytomegalovirus (MCMV). The aim of this study was to localize Cmv1 more precisely in relation to other NKC loci by generating a high-resolution genetic map of the region. We have analyzed 1250 backcross mice comprising panels of 700 (BALB/c x C57BL/6J)F1 x BALB/c and 550 (A/J x C57BL/6J)F1 x A/J progeny. A total of 25 polymorphic genes or microsatellite markers were analyzed over a region of 10 map units from D6Mit134 to D6Mit59. The Cmv1 phenotypes of mice recombinant in this interval were tested by infection with MCMV. The results obtained indicate that the functionally important NKC region is a tightly linked cluster of loci spanning at least 0.4 map units. Furthermore, Cmv1 maps distal to, but very closely linked to, the Ly49 multigene family (< 0.2 map units), suggesting that MCMV resistance may be conferred by MHC class I-specific NK cell receptors.

Inhibition of natural killer cells by a cytomegalovirus MHC class I homologue in vivo

Herpesviruses, such as murine and human cytomegalovirus (MCMV and HCMV), can establish a persistent infection within the host and have diverse mechanisms as protection from host immune defences. Several herpesvirus genes that are homologous to host immune modulators have been identified, and are implicated in viral evasion of the host immune response. The discovery of a viral major histocompatibility complex (MHC) class I homologue, encoded by HCMV, led to speculation that it might function as an immune modulator and disrupt presentation of peptides by MHC class I to cytotoxic T cells. However, there is no evidence concerning the biological significance of this gene during viral infection. Recent analysis of the MCMV genome has also demonstrated the presence of a MHC class I homologue. Here we show that a recombinant MCMV, in which the gene encoding the class I homologue has been disrupted, has severely restricted replication during the acute stage of infection compared with wild-type MCMV. We demonstrate by in vivo depletion studies that natural killer (NK) cells are responsible for the attenuated phenotype of the mutant. Thus the viral MHC class I homologue contributes to immune evasion through interference with NK cell-mediated clearance.

The natural killer gene complex: a genetic basis for understanding natural killer cell function and innate immunity

The natural killer gene complex encodes proteins, some of which are structurally unrelated, that impact on NK-cell function. Detailed analyses have indicated that these molecules are involved in NK-cell recognition, activation, and inhibition. The importance of this genomic region is highlighted by studies indicating that NKC-associated genes significantly influence NK cell-mediated innate host defense against life-threatening pathogens and that the NKC is conserved among diverse species. Thus, further elucidation of the NKC and its gene products will provide a genetic basis for understanding innate immunity and NK-cell activity at the molecular level.

Effect of host genotype in determining the relative roles of natural killer cells and T cells in mediating protection against murine cytomegalovirus infection

The influence of host genotype on the relative importance of T cell subsets and natural killer (NK) cells in controlling murine cytomegalovirus (MCMV) replication has been investigated. Genetically susceptible BALB/c and A/J, moderately resistant C57BL/10, and resistant CBA/CaH mouse strains were treated with monoclonal antibodies (MAb) to the CD4 and CD8 markers and the extent of MCMV replication in major target tissues was determined. Both mouse strain-specific and tissue-specific effects were observed. CBA/CaH and C57BL/10 mice were found not to require CD4+ or CD8+ T cells for control of MCMV replication in the spleen or liver. In contrast, in A/J mice, as well as BALB/c mice, the CD8+ T cell population was primarily responsible for the clearance of virus from these tissues. However, in all strains of mice, CD4+ T cells were required for delayed type hypersensitivity and antibody responses, and for virus clearance in the salivary glands. The dependence of mice with the BALB genetic background on CD8+ T cells for limitation of acute MCMV infection was found to be negated in the BALB.B6-Cmv1(r) congenic strain, in which an effective NK cell response has been generated through the introduction of the resistant Cmv1(r) allele from C57BL/6 mice. Depletion of NK cells in the BALB.B6-Cmv1(r) strain using anti-NK1.1 MAb restored the role of CD8+ T cells in mediating viral clearance. These analyses demonstrate that some, but not all, strains of mice use CD8+ T cells to control MCMV replication and that even when CD8+ T cell-dependence exists, this can be circumvented by an appropriate NK cell response.

Effect of natural sequence variation at the H-2Ld-restricted CD8+ T cell epitope of the murine cytomegalovirus ie1-encoded pp89 on T cell recognition

The amino acid sequence YPHFMPTNL of pp89, the ie1-encoded product of murine cytomegalovirus (MCMV; Smith strain), constitutes an immunodominant T cell epitope recognized in association with H-2Ld. Nucleotide sequencing of MCMV isolates derived from wild mice identified variation between amino acids 147-192 of pp89 in 19 of 27 isolates, including the region encompassing the CTL epitope (amino acid residues 168-176). Four groups of isolates with naturally occurring variant sequences for the CTL epitope were defined: (1) YPHFMPPNL; (2) YPHFMPPSL; (3) YPHFIPPSL; and (4) YLDFMPPNL. The remaining isolates, and the laboratory strains K181 and Vancouver, showed complete identity with the Smith strain. Polyclonal pp89 (Smith strain)-specific CTL only weakly recognized target cells infected with MCMV from most variant groups. No lysis of cells infected with isolate N1 from group 4 was detected. Analyses of cross-reactive recognition of YPHFMPTNL peptide-coated targets by CTL primed with variant MCMV isolates showed that the group 2 and 3 isolates, G4 and K6, respectively, but not the group 4 isolate N1, elicited CTL that exhibited a cross-reactive response. Furthermore, while the group 2 and 3 isolates G4 and K6 were able to prime CTL responses that displayed reactivity to homologous pp89 variant nonapeptides, the group 4 isolate N1 failed to do so. Finally, while immunization of mice with the nonapeptide YPHFMPTNL conferred significant protection against the laboratory strain K181 [correction of Kl81], no evidence of protection was observed for the group 2 and 4 variants G4 and N1, respectively. These observations raise the possibility that clinical isolates of HCMV may also differ in sequence from potential vaccine strains at immunodominant epitopes for CD8+ T cells thus reducing the efficacy of vaccination.

Recombinant polyepitope vaccines for the delivery of multiple CD8 cytotoxic T cell epitopes

Development of epitope-based CD8 alpha beta CTL vaccines requires effective strategies for codelivery of large numbers of individual epitopes. We have designed an artificial "polyepitope" protein containing 10 contiguous minimal CTL epitopes, which were restricted by five MHC alleles and derived from five viruses, a parasite, and a tumor model. A recombinant vaccinia virus coding for this protein was capable of inducing MHC-restricted primary CTL responses to all 10 epitopes. Mice immunized with this recombinant vaccinia showed protection against murine cytomegalovirus, Sendai virus, and a tumor model. This simple generic approach to multiepitope delivery should find application in CTL-based vaccine design.

Recombinant polyepitope vaccines for the delivery of multiple CD8 cytotoxic T cell epitopes

Development of epitope-based CD8 alpha beta CTL vaccines requires effective strategies for codelivery of large numbers of individual epitopes. We have designed an artificial "polyepitope" protein containing 10 contiguous minimal CTL epitopes, which were restricted by five MHC alleles and derived from five viruses, a parasite, and a tumor model. A recombinant vaccinia virus coding for this protein was capable of inducing MHC-restricted primary CTL responses to all 10 epitopes. Mice immunized with this recombinant vaccinia showed protection against murine cytomegalovirus, Sendai virus, and a tumor model. This simple generic approach to multiepitope delivery should find application in CTL-based vaccine design.

Assessment of antigenicity and genetic variation of glycoprotein B of murine cytomegalovirus

An analysis of linear antibody-binding sites of the glycoprotein B (gB) molecule of murine cytomegalovirus (MCMV) and of genetic variation within these regions was performed. To achieve this, a series of overlapping fragments spanning the entire coding sequence of the gB gene of the K181 strain of MCMV was expressed in E. coli as fusion proteins with glutathione S-transferase (GST) using the pGEX expression system. Four antibody-binding regions were mapped to locations spanning amino acid residues 17-79 (BS), 155-278 (BE2), 809-926 (SS) and 347-508 (BB and EE), based on reactivity in Western blot analysis of GST-gB fusion proteins with murine polyclonal antiserum raised against MCMV. Only the antibody-binding region BE2 (155-278) elicited an antiserum that exhibited complement-dependent neutralizing activity, and immunization of mice with the fusion protein BE2 led to moderate but significant reductions in the level of MCMV replication in the spleen. Polyclonal antisera raised against the GST-gB fusion proteins detected purified virion proteins of 105 kDa (anti-BS and anti-BE2) and 52 kDa (anti-SS) and are therefore likely to recognize the N-terminal and C-terminal portions of the gB molecule, respectively. The antibody-binding region within amino acid residues 17-79 was found to be MCMV strain-specific, whereas antibody-binding regions within residues 155-278 and 809-926 were found to be conserved among MCMV field isolates. Comparative sequence analysis of the corresponding regions of MCMV gB revealed a level and extent of sequence of sequence heterogeneity consistent with these findings.

DNA sequence and transcriptional analysis of the glycoprotein M gene of murine cytomegalovirus

We have characterized the gene encoding the murine cytomegalovirus (MCMV) homologue of the human cytomegalovirus (HCMV) UL100 open reading frame (ORF) that encodes the HCMV glycoprotein M (gM) molecule. It was identified based on its collinearity with MCMV homologues of the HCMV UL99, UL102, UL103 and UL104 ORFs which lie in the HindIII G fragment of the K181 strain of MCMV. Sequencing of a 2.3 kb EcoRI-BamHI subfragment of the EcoRI G fragment adjacent to the EcoRI A fragment revealed the presence of the complete MCMV gM ORF and two incomplete ORFs, which corresponded to homologues of HCMV UL99 and UL102. The MCMV gM ORF consists of 1059 nucleotides and is expressed as a 1.2 kb transcript at late times post-infection. To precisely characterize the gM transcript, the 5' and 3' ends were mapped. It was found that the transcript initiates at nucleotides 740 or 745, and that the site of polyadenylation at nucleotide 1961 occurs downstream of the second potential polyadenylation signal located at nucleotide 1934. Based on these findings the MCMV gM is predicted to consist of 353 residues and when compared with HCMV gM has a 47% level of identity. Of great interest is the finding that the MCMV gM amino acid sequence is completely conserved among six isolates of MCMV that had been shown to exhibit considerable variation both in the MCMV glycoprotein B and the immediate-early 1 gene-encoded pp89 molecule. Thus, this glycoprotein appears to be antigenically conserved.

Genetic mapping of Cmv1 in the region of mouse chromosome 6 encoding the NK gene complex-associated loci Ly49 and musNKR-P1

The Cmv1 resistance gene controls splenic replication of murine cytomegalovirus (MCMV) and confers natural killer (NK) cell-mediated resistance to otherwise lethal infection. The Cmv1 phenotypes of 13 inbred mouse strains have been assessed, and it was found that the Cmv1r resistance phenotype was restricted to the C57BL/6J and Ma/MyJ strains. We have further analyzed the linkage of Cmv1 to the NK gene complex (NKC) mapping to distal mouse chromosome 6 in 99 (BALB/c x C57BL/6J)F1 x BALB/c backcross mice using cloned gene probes and microsatellite markers from this region. No recombinants were observed between Cmv1 and the NKC-associated Ly49 and musNKR-P1 multigene families, nor the Kap locus, nor with 7 microsatellite markers, indicating that Cmv1 is closely linked (< 1 cM) to all of these markers. Analysis of the genotype of the MCMV-susceptible BXD8 RI strain around the NKC region revealed that it had C57BL/6J alleles at microsatellite markers immediately proximal and distal to Cmv1. This suggests that the Cmv1s phenotype of this strain is due to a germ-line mutation. Thus, the close linkage of Cmv1 to the Ly49 and musNKR-P1 multigene families suggests that it may represent an NK cell recognition structure encoded in the NKC region.

Induction of protective cytotoxic T cells to murine cytomegalovirus by using a nonapeptide and a human-compatible adjuvant (Montanide ISA 720)

The use of synthetic peptides representing cytotoxic T-cell (CTL) epitopes for human vaccination requires the identification of a suitable adjuvant formulation. A single immunization with Montanide ISA720/tetanus toxoid/YPHFMPTNL protected mice against murine cytomegalovirus and induced epitope-specific CTL. Such formulations will find application in peptide-based CTL anti-viral vaccines.

Mapping and transcriptional analysis of the murine cytomegalovirus homologue of the human cytomegalovirus UL103 open reading frame

Murine cytomegalovirus (MCMV) has been widely used as a model for human cytomegalovirus (HCMV) infection since the two viruses share many biological and pathogenic properties. To further evaluate the similarities in the genomes of these viruses we have sought to identify MCMV genes that encode immunoreactive proteins by screening a lambda gt11 cDNA library with anti-MCMV monoclonal antibodies (MAb). Clone 430, identified on the basis of its reactivity with the MAb 6/20/1, was completely sequenced and shown to encode the MCMV homologue of the HCMV UL103 open reading frame (ORF). This ORF is encoded by a 1.1-kb transcript that is expressed at late times following infection. Sequencing of the cDNA also revealed the presence of two incomplete ORFs encoding the homologues of the HCMV UL102 and 104 ORFs. These ORFs are encoded by a 2.4-kb leaky late transcript and a 3.0-kb late transcript, respectively. Together these ORFs constitute part of a block of genes conserved in all herpesviruses, and in the MCMV and HCMV genomes this block of genes shows striking positional homology. The high degree of homology between HCMV and MCMV ORFs, at both the nucleotide and amino acid levels, plus the growing evidence of conservation in gene order between the two genomes suggests that the two viruses are very similar at the molecular level.

Identification, sequencing and expression of the glycoprotein L gene of murine cytomegalovirus

DNA sequence analysis of the genome of the Smith strain of murine cytomegalovirus (MCMV) revealed an open reading frame (ORF) with amino acid sequence identity to glycoprotein L (gL) of other herpesviruses. The ORF is 822 bp in size and has the capacity to encode a protein of 274 amino acids. It has significant identity with the gL genes of human CMV and human herpesvirus 6. The coding sequence of the gL gene of MCMV strain K181 was also determined, and expressed in Escherichia coli as a fusion protein with glutathione S-transferase using the pGEX expression system. Two antibody-binding regions were identified on the basis of the reactivity of a series of truncated gL constructs with anti-MCMV antibodies. One was mapped to residues 1 to 38 and the other between residues 230 and 274. Polyclonal antibodies specific to gL were raised against the full-length gL fusion protein. The antisera were shown to react with a 46K protein present in purified virions by Western blotting. Treatment of purified virions with endoglycosidase-H or -F resulted in reductions in M(r) of the 46K species to 42K and 31K, respectively. The antisera did not exhibit any neutralizing activity in a plaque reduction assay.

Expression of the glycoprotein H of murine cytomegalovirus and identification of an N-terminal antibody-binding region

A series of overlapping fragments spanning the entire coding sequence of the gH gene of murine cytomegalovirus (MCMV) were expressed in Escherichia coli as fusion proteins with glutathione S-transferase (GST) using the pGEX expression system. A region of antibody-binding was mapped to the NH2-terminus of glycoprotein H (gH) between amino acid residues 26 and 90 on the basis of the reactivity of GST-gH fusion proteins with polyclonal antibodies to MCMV in Western blot analysis. Antibodies to gH were generated in mice immunized with the GST-gH fusion protein SK and shown to react with an 87-kDa polypeptide present in virion particles which was conserved in MCMV isolates obtained from diverse locations. They also recognized the gH protein in MCMV-infected cells, as well as gH expressed in Chinese Hamster Ovary cells. The antibodies to gH had a significant ELISA titer but no neutralizing activity in vitro. The antibody response to the GST-gH fusion protein did not modify the level of MCMV replication in the spleens of mice.

Identification and characterization of a murine cytomegalovirus gene with homology to the UL25 open reading frame of human cytomegalovirus

Monoclonal antibody 1B4, previously shown to be protective in vivo and to cross-react with both virally encoded and normal host cell proteins, was used to screen a lambda gt11 cDNA derived from mRNA harvested from mouse embryo fibroblasts 24 hr after infection with murine cytomegalovirus (MCMV). A 700-bp cDNA was identified representing the 5'terminus of a 2460-bp open reading frame (ORF) with significant homology to the human cytomegalovirus UL25 ORF. The UL25 ORF of MCMV potentially encodes an 820 amino acid viral tegument protein with an estimated molecular weight of approximately 90 kDa. Amino acid homology with eukaryotic nucleolins was identified in the acidic N-terminal third of the MCMV UL25 proteins, suggesting that the protein may be involved in transcriptional activation or interactions with chromatin. Northern analysis and S1 nuclease data indicated that the gene is expressed late in infection as an approximately 3-kb transcript and that expression is dependent on viral DNA replication. An epitope recognized by MAb 1B4 was identified using recombinant pGEX plasmids expressing fusion proteins representing the N-terminal region of the MCMV UL25 protein. The identification of the MCMV UL25 ORF as a member of the CMV-specific UL25/UL35 gene family provides an opportunity for the investigation of the role these genes and their products in CMV pathogenesis in an animal model.

A murine cytomegalovirus-neutralizing monoclonal antibody exhibits autoreactivity and induces tissue damage in vivo

The autoreactivity of murine cytomegalovirus (MCMV)-neutralizing monoclonal antibody (mAb) AC1 was examined in vitro and in vivo. Both mAb AC1 and a human antiserum reactive with U1-small nuclear ribonucleoprotein (U1-snRNP) stained uninfected mouse embryo fibroblasts (MEF) in a speckled nuclear pattern and reacted with 70,000 molecular weight (MW) MEF nuclear antigens by immunoblotting, suggesting that mAb AC1 cross-reacted with the 70,000 MW component of U1-snRNP. However, only mAb AC1 cross-reacted with an additional epithelial cytoplasmic autoantigen present in cultured HEp2 cells. On tissue sections from uninfected mice, mAb AC1 predominantly reacted with a component of central and peripheral nervous systems, although cross-reactivity with the stratum spinosum of the skin and the outer sheath of hair follicles was also observed. Immunoblotting revealed that mAb AC1 reacted with phosphorylated epitopes present on a 98,000 MW MCMV structural protein and the 200,000 MW mouse neurofilament protein (NFP). Treatment of uninfected mice with mAb AC1 resulted in a severe interstitial pneumonia with greatly thickened and congested alveolar septa. Severe oedema of the hypodermis and a mild mesangial proliferative glomerulonephritis were also observed. These results demonstrate that a mAb reacting with a MCMV structural phosphoprotein which can protect mice against the dissemination of MCMV, can also promote the development of autoimmune disease. Therefore, the production of such cross-reactive antibodies may be an important mechanism in the development of autoimmunity following viral infection.

Molecular and biological characterization of new strains of murine cytomegalovirus isolated from wild mice

Studies of the prevalence of antibody to murine cytomegalovirus (MCMV) in free-living wild mice (Mus domesticus) trapped in diverse regions of Australia and on a sub-Antarctic island indicated that 90% of 468 mice had serum antibody to MCMV. Twenty-six field isolates of MCMV were plaque-purified from salivary gland extracts of representative seropositive mice. These isolates varied considerably in their ability to replicate in the salivary glands of weanling BALB/c mice with 9 of 15 failing to reach significant titres in this organ and the titres of the remaining 6 strains varying by at least 100-fold. The high frequency of restriction fragment length polymorphisms observed suggests widespread genetic heterogeneity exists among the strains. This observation was mirrored at the polypeptide level by Western blot analyses with polyclonal antisera to MCMV. The isolation in this study of four genetically distinct strains of MCMV from a single wild mouse and several strains from other individual mice demonstrates that multiple infections with MCMV may be commonplace in wild mice.

The effect of the Cmv-1 resistance gene, which is linked to the natural killer cell gene complex, is mediated by natural killer cells

The resistance of mice to lethal infection by murine CMV (MCMV) is under complex host genetic control with contributions from both H-2 and non-H-2 genes. We have previously shown that an autosomal, non-MHC encoded gene, Cmv-1, controls MCMV replication in the spleen. We have investigated the mechanism by which the Cmv-1 resistance gene confers protection against MCMV infection. Using H-2 compatible irradiation bone marrow chimeras, the enhanced resistance to MCMV infection that is associated with the Cmv-1l allele in the C57BL background was shown to be mediated by an irradiation-sensitive bone marrow-derived cell population, or a factor produced by these cells. The lack of correlation between serum IFN titers and the strain distribution pattern of Cmv-1 in CXB recombinant inbred mouse strains suggests that IFN does not mediate resistance conferred by this gene. Similarly, the lack of effect of in vivo depletion of mature CD4+ and CD8+ T cells on virus replication in C57BL/6J mice indicates that T cells are unlikely to be involved. In contrast, in vivo depletion of NK cells by injection of the anti-NK1.1 mAb PK136 abrogated restricted splenic virus replication in C57BL/6J----BALB.B chimeric mice and in the Cmv-1l CXB strains. These data indicate that the effect of the Cmv-1 gene is mediated by NK cells. The significant augmentation in NK cell activity after MCMV infection of the susceptible Cmv-1h strains (BALB/cBy), CXBG/By, CXBH/By, CXBI/By, and CXBK/By) indicates the existence in these mice of NK cells that are functionally and phenotypically distinct from those in Cmv-1l strains. NK cells present in the Cmv-1h strains are unable to restrict efficiently splenic MCMV replication in vivo, possibly due to a lack of specificity for virus-infected target cells. Finally, flow cytometric analysis of NK1-1 expression in CXB and BXD RI mice together with MCMV replication studies in the BXD RI strains indicate that Cmv-1 is closely linked to NK1.1 and other loci that reside on a distal segment of murine chromosome 6 in a region that has recently been defined as the natural killer complex.

The effect of the Cmv-1 resistance gene, which is linked to the natural killer cell gene complex, is mediated by natural killer cells

The resistance of mice to lethal infection by murine CMV (MCMV) is under complex host genetic control with contributions from both H-2 and non-H-2 genes. We have previously shown that an autosomal, non-MHC encoded gene, Cmv-1, controls MCMV replication in the spleen. We have investigated the mechanism by which the Cmv-1 resistance gene confers protection against MCMV infection. Using H-2 compatible irradiation bone marrow chimeras, the enhanced resistance to MCMV infection that is associated with the Cmv-1l allele in the C57BL background was shown to be mediated by an irradiation-sensitive bone marrow-derived cell population, or a factor produced by these cells. The lack of correlation between serum IFN titers and the strain distribution pattern of Cmv-1 in CXB recombinant inbred mouse strains suggests that IFN does not mediate resistance conferred by this gene. Similarly, the lack of effect of in vivo depletion of mature CD4+ and CD8+ T cells on virus replication in C57BL/6J mice indicates that T cells are unlikely to be involved. In contrast, in vivo depletion of NK cells by injection of the anti-NK1.1 mAb PK136 abrogated restricted splenic virus replication in C57BL/6J----BALB.B chimeric mice and in the Cmv-1l CXB strains. These data indicate that the effect of the Cmv-1 gene is mediated by NK cells. The significant augmentation in NK cell activity after MCMV infection of the susceptible Cmv-1h strains (BALB/cBy), CXBG/By, CXBH/By, CXBI/By, and CXBK/By) indicates the existence in these mice of NK cells that are functionally and phenotypically distinct from those in Cmv-1l strains. NK cells present in the Cmv-1h strains are unable to restrict efficiently splenic MCMV replication in vivo, possibly due to a lack of specificity for virus-infected target cells. Finally, flow cytometric analysis of NK1-1 expression in CXB and BXD RI mice together with MCMV replication studies in the BXD RI strains indicate that Cmv-1 is closely linked to NK1.1 and other loci that reside on a distal segment of murine chromosome 6 in a region that has recently been defined as the natural killer complex.

Identification of the glycoprotein H gene of murine cytomegalovirus

Partial sequencing of the HindIII C fragment of murine cytomegalovirus (MCMV) revealed an open reading frame of 2172 nucleotides in length encoding a 724 amino acid protein with a predicted Mr of 80.4K. Analysis of the predicted amino acid sequence revealed homology with glycoprotein H (gH) from a number of other herpesviruses. MCMV gH showed strongest amino acid identity with human (H) CMV and human herpesvirus 6 gH, and less identity with the gH protein sequences of Epstein-Barr virus, varicella-zoster virus and herpes simplex virus type 1. The greatest identity between MCMV and HCMV gH occurs in the C-terminal region. The MCMV gH is characterized by having a 14 amino acid signal sequence, a 23 amino acid transmembrane region, a seven amino acid positively charged cytoplasmic anchor sequence and eight putative N-linked glycosylation sites. Comparison of MCMV gH with that of HCMV indicates that there are 12 conserved cysteine residues and three conserved potential N-linked glycosylation sites.

Structure/function studies of murine interferon-alpha 1 using site-directed mutagenesis followed by in vitro synthesis

Site-directed in vitro mutagenesis followed by in vitro transcription and translation has been used to study structure/function relationships for murine interferon-alpha 1 (MuIFN-alpha 1). The mature form of the MuIFN-alpha 1 protein was expressed as well as analogue forms with amino acid substitutions at positions 33, 71, 72, 123 and 133. These positions were chosen on the basis of known human interferon-alpha structure/function relationships. Biological assays for antiviral activity on murine cells and natural killer cell activation have been performed for each of the proteins produced. The data obtained have been interpreted in the light of previous human and murine interferon-alpha structure/function work and the recently published three-dimensional structure of murine type I interferon.

Cmv-1, a genetic locus that controls murine cytomegalovirus replication in the spleen

The genetic basis of the control of acute splenic MCMV infection was studied after intraperitoneal inoculation of the virus. Classical Mendelian analyses using C57BL/6 (resistant) and BALB/c (susceptible) parental strains disclosed an autosomal dominant non-H-2 gene that regulates splenic virus replication. The probable location of this gene, to which we have assigned the symbol Cmv-1, is on chromosome 6 as defined by the strain distribution pattern of splenic MCMV replication in CXB recombinant inbred mice. Although there is a similar hierarchy of resistance to MCMV and HSV-1 with respect to the C57BL and BALB genetic backgrounds, the strain distribution pattern of HSV-1 replication in recombinant inbred mice suggests that Cmv-1 is not involved in restricting the spread of this virus. This is the first clear identification of a non-H-2 gene regulating the magnitude of MCMV infection. Elucidation of the function of this gene may be a fundamental step towards understanding the control of systemic CMV infection.

Murine cytomegalovirus binds reversibly to mouse embryo fibroblasts: implications for quantitation and explanation of centrifugal enhancement

In a study of the infection of mouse embryo fibroblasts with murine cytomegalovirus (MCMV), we found that plaque number is directly related to virus concentration and not to the total amount of virus contained in the inoculum. These results suggested that virus binding was reversible and that during infection a binding equilibrium is established which limits the amount of bound virus. Further analysis revealed three categories of plaque based on reversibility after virus adsorption. One group was removed simply by washing cell monolayers after virus removal. A second group of plaques was lost gradually with time, giving complete reversal after 5 min at 37 degrees C. The rate of reversal was temperature dependent, and probably represented true virus dissociation. The final group was irreversible plaques, the number of which increased with increasing infection time. The number of reversible plaques remained constant with time of infection, and represented about 70% of the total plaques after 1 h of virus adsorption. Centrifugation of the virus inoculum onto the fibroblast monolayer at 1000 X g increased plaque numbers up to 100-fold, but had little effect on plaque number when carried out after the virus inoculum was removed. In contrast centrifugation increased the number of reversible plaques, suggesting an increase in the number of virus particles attached to the cell monolayers. We suggest that centrifugation enhances MCMV infection by three mechanisms related to reversibility of binding: (1), it increases the rate of virus association; (2), it decreases the rate of dissociation; (3), by increasing the length of time each virus particle is bound it increases the probability of virus being taken into the cell.

Direct role of viral hemagglutinin in B-cell mitogenesis by influenza viruses

The mitogenic activity of influenza virus is a function of the hemagglutinin (HA) molecule. Purified HA is mitogenic for murine B lymphocytes but not T lymphocytes. Furthermore, like the intact virus, HA of the H2 (but not H3) subtype is mitogenic only for B cells expressing the class II major histocompatibility complex glycoprotein I-E. Since virus bearing uncleaved HA is as mitogenic as virus bearing cleaved HA, the membrane fusion activity of the HA molecule is not involved.

Relationship between mitogenic activity of influenza viruses and the receptor-binding specificity of their hemagglutinin molecules

The relationship between the mitogenic activity of influenza type A viruses for murine B lymphocytes and the receptor-binding specificity of their hemagglutinin was examined. Receptor-binding specificity was determined by the ability of the virus to agglutinate erythrocytes that had been sialidase treated and then enzymatically resialylated to contain sialyloligosaccharides with defined sequences. Distinct differences in receptor-binding specificity were observed between strongly and weakly mitogenic viruses of the H3 subtype, with strong mitogenic activity correlating with the ability of the virus to recognize the sequence N-glycolylneuraminic acid alpha 2,6 galactose (NeuGc alpha 2,6Gal). Viruses isolated early in the evolution of the H3 subtype (from 1968 to 1971) are relatively weak mitogens and recognize the sequence N-acetylneuraminic acid alpha 2,6 galactose (NeuAc alpha 2,6Gal) but not NeuGc alpha 2,6Gal. H3 viruses isolated since 1972 are strongly mitogenic, and these viruses recognize both NeuGc alpha 2,6Gal and NeuAc alpha 2,6Gal. The amino acid substitution of Tyr for Thr at residue 155 of HA1 may be critical to this change in receptor-binding specificity and mitogenic activity of the later H3 viruses. Horse serum-resistant variants of H3 viruses, which bind preferentially to the sequence NeuAc alpha 2,3Gal, are poorly mitogenic. Differences were also observed between the receptor-binding specificity of the strongly mitogenic H3 viruses and viruses of the H2 and H6 subtypes, the mitogenic activity of which is limited to strains of mice that express the class II major histocompatibility complex glycoprotein I-E. The results indicate that the receptor-binding specificity of the hemagglutinin plays a critical role in determining the mitogenic activity of influenza viruses.

Influenza viruses as lymphocyte mitogens. II. Role of I-E molecules in B cell mitogenesis by influenza A viruses of the H2 and H6 subtypes

Influenza A viruses of the H2 and H6 subtypes behave as T cell-independent B cell mitogens for lymphocytes from strains of mice that express the class II MHC glycoprotein I-E (Ia.7+ haplotypes). We have examined the role of I-E molecules in mitogenesis by these viruses. Lymphocytes from (Ia.7+ X Ia.7-)F1 hybrid strains that express lower levels of I-E antigen than homozygous Ia.7+ strains showed a level of response to H2 and H6 influenza viruses that was intermediate between the high response of the Ia.7+ parent and the low response of the Ia.7- parent. The mitogenic response of H-2k lymphocytes to these viruses was completely inhibited by low concentrations of anti-I-Ek monoclonal antibody that had no effect on B cell proliferation induced by LPS or by influenza A virus of the H3 subtype. Furthermore, incubation of H-2k spleen cells with high concentrations of H2 (but not H3) influenza viruses substantially inhibited the binding of radio-labeled anti-I-Ek, but not anti-I-Ak, monoclonal antibody. Cell mixing experiments indicated that expression of I-E by the B cells was critical to the mitogenic response, whereas I-E expression by accessory cells may not be necessary. The data support a model in which B cell mitogenesis by these viruses results from direct binding of the viruses to I-E molecules on B lymphocytes.

Influenza viruses as lymphocyte mitogens. II. Role of I-E molecules in B cell mitogenesis by influenza A viruses of the H2 and H6 subtypes

Influenza A viruses of the H2 and H6 subtypes behave as T cell-independent B cell mitogens for lymphocytes from strains of mice that express the class II MHC glycoprotein I-E (Ia.7+ haplotypes). We have examined the role of I-E molecules in mitogenesis by these viruses. Lymphocytes from (Ia.7+ X Ia.7-)F1 hybrid strains that express lower levels of I-E antigen than homozygous Ia.7+ strains showed a level of response to H2 and H6 influenza viruses that was intermediate between the high response of the Ia.7+ parent and the low response of the Ia.7- parent. The mitogenic response of H-2k lymphocytes to these viruses was completely inhibited by low concentrations of anti-I-Ek monoclonal antibody that had no effect on B cell proliferation induced by LPS or by influenza A virus of the H3 subtype. Furthermore, incubation of H-2k spleen cells with high concentrations of H2 (but not H3) influenza viruses substantially inhibited the binding of radio-labeled anti-I-Ek, but not anti-I-Ak, monoclonal antibody. Cell mixing experiments indicated that expression of I-E by the B cells was critical to the mitogenic response, whereas I-E expression by accessory cells may not be necessary. The data support a model in which B cell mitogenesis by these viruses results from direct binding of the viruses to I-E molecules on B lymphocytes.

Mitogenic activity of influenza virus and haemagglutinin

Influenza A viruses behave as T cell-independent B cell mitogens in vitro. The magnitude of the proliferation induced varies with the haemagglutinin subtype of the virus, the order being H2 greater than H6 greater than H3 greater than H1 for Balb/c mice. H3 viruses are equally mitogenic for all strains of mice tested. In contrast, the mitogenic response to H2 and H6 viruses is controlled by the I-E subregion of the major histocompatibility complex. These viruses are mitogenic only for strains of mice that express surface I-E antigen (haplotypes a, d, k, p, r), and not for haplotypes b, f, q, s, which fail to synthesize a normal E alpha chain and do not express surface I-E antigen. Mitogenesis by H2 and H6 viruses may involve direct interaction of virus with I-E molecules on the B lymphocyte or an accessory cell, whereas mitogenesis by H3 viruses does not appear to involve I-E.

Influenza viruses as lymphocyte mitogens. I. B cell mitogenesis by influenza A viruses of the H2 and H6 subtypes is controlled by the I-E/C subregion of the major histocompatibility complex

Influenza A viruses of the H2, H3, and H6 subtypes function as T cell-independent B cell mitogens for lymphocytes from BALB/c mice. Lymphocytes from C57BL/10 mice, however, undergo mitogenesis only in response to H3 viruses. The failure of C57BL/10 lymphocytes to respond to H2 and H6 viruses was shown not to reflect a difference in dose-response profile or kinetics of the response, nor was it due to the activity of suppressor T cells. Experiments with congenic and recombinant strains of mice established that mitogenic responsiveness to H2 and H6 viruses is linked to the major histocompatibility complex, and is controlled by a gene located in the I-E/C subregion. Furthermore, responsiveness was shown to correlate with the expression of surface I-E antigen, being positive for mouse strains that express I-E antigen (haplotypes a, d, k, p, r) and negative for strains that do not (haplotypes b, f, q, s). The data suggest that influenza A viruses of the H2 and H6 subtypes may interact directly with I-E molecules on the surface of B cells or possibly on an accessory cell. Because mitogenesis by H3 viruses is not I-E dependent, it appears that influenza A viruses stimulate B cell mitogenesis by at least two different mechanisms.

Influenza viruses as lymphocyte mitogens. I. B cell mitogenesis by influenza A viruses of the H2 and H6 subtypes is controlled by the I-E/C subregion of the major histocompatibility complex

Influenza A viruses of the H2, H3, and H6 subtypes function as T cell-independent B cell mitogens for lymphocytes from BALB/c mice. Lymphocytes from C57BL/10 mice, however, undergo mitogenesis only in response to H3 viruses. The failure of C57BL/10 lymphocytes to respond to H2 and H6 viruses was shown not to reflect a difference in dose-response profile or kinetics of the response, nor was it due to the activity of suppressor T cells. Experiments with congenic and recombinant strains of mice established that mitogenic responsiveness to H2 and H6 viruses is linked to the major histocompatibility complex, and is controlled by a gene located in the I-E/C subregion. Furthermore, responsiveness was shown to correlate with the expression of surface I-E antigen, being positive for mouse strains that express I-E antigen (haplotypes a, d, k, p, r) and negative for strains that do not (haplotypes b, f, q, s). The data suggest that influenza A viruses of the H2 and H6 subtypes may interact directly with I-E molecules on the surface of B cells or possibly on an accessory cell. Because mitogenesis by H3 viruses is not I-E dependent, it appears that influenza A viruses stimulate B cell mitogenesis by at least two different mechanisms.

Influenza viruses are T cell-independent B cell mitogens

UV-inactivated influenza virus A strains of subtypes H1, H2, H3, and H6 were shown to be mitogenic for unprimed splenic lymphocytes from BALB/c mice. Representative viruses of these four subtypes all behaved as T cell-independent B cell mitogens. The magnitude of the proliferative response was determined by the subtype of the hemagglutinin molecule: H2 and H6 viruses were the most potent mitogens, and H3 viruses were moderately mitogenic, whereas H1 viruses induced only low, but significant, levels of proliferation. Mitogenesis was inhibited by antiviral sera and by monoclonal antibodies directed against hemagglutinin.