Ly6 family proteins in neutrophil biology

The murine Ly6 complex was identified 35 years ago using antisera to lymphocytes. With advances in mAb development, molecular cloning, and genome sequencing, >20 structurally related genes have been identified within this complex on chromosome 15. All members of the Ly6 family and their human homologues share the highly conserved LU domain and most also possess a GPI anchor. Interestingly, many Ly6 proteins are expressed in a lineage-specific fashion, and their expression often correlates with stages of differentiation. As a result, Ly6 proteins are frequently used as surface markers for leukocyte subset identification and targets for antibody-mediated depletion. Murine neutrophils display prominent surface expression of several Ly6 proteins, including Ly6B, Ly6C, and Ly6G. Although the physiology of most Ly6 proteins is not well understood, a role in neutrophil functions, such as migration, is recognized increasingly. In this review, we will provide an overview of the Ly6 complex and discuss, in detail, the specific Ly6 proteins implicated in neutrophil biology.

Detection of prokaryotic mRNA signifies microbial viability and promotes immunity

Live vaccines have long been known to trigger far more vigorous immune responses than their killed counterparts. This has been attributed to the ability of live microorganisms to replicate and express specialized virulence factors that facilitate invasion and infection of their hosts. However, protective immunization can often be achieved with a single injection of live, but not dead, attenuated microorganisms stripped of their virulence factors. Pathogen-associated molecular patterns (PAMPs), which are detected by the immune system, are present in both live and killed vaccines, indicating that certain poorly characterized aspects of live microorganisms, not incorporated in dead vaccines, are particularly effective at inducing protective immunity. Here we show that the mammalian innate immune system can directly sense microbial viability through detection of a special class of viability-associated PAMPs (vita-PAMPs). We identify prokaryotic messenger RNA as a vita-PAMP present only in viable bacteria, the recognition of which elicits a unique innate response and a robust adaptive antibody response. Notably, the innate response evoked by viability and prokaryotic mRNA was thus far considered to be reserved for pathogenic bacteria, but we show that even non-pathogenic bacteria in sterile tissues can trigger similar responses, provided that they are alive. Thus, the immune system actively gauges the infectious risk by searching PAMPs for signatures of microbial life and thus infectivity. Detection of vita-PAMPs triggers a state of alert not warranted for dead bacteria. Vaccine formulations that incorporate vita-PAMPs could thus combine the superior protection of live vaccines with the safety of dead vaccines.

Conservation of CD1 protein expression patterns in the chicken

The CD1 molecules are cell-surface proteins that bind and present foreign lipids and glycolipids to T cells in a manner similar to the MHC system. While the mammalian CD1 antigen presentation pathway is well characterized, little is known about CD1 in non-mammalian vertebrates. Previous studies have identified two CD1 homologues in the chicken. We developed a monoclonal antibody designated NL1-1.A1 specific for the chCD1-1 isoform and have used this to characterize CD1 expression in tissues and cells of normal adult and embryonic chickens. The chCD1-1 isoform is expressed on a high proportion of cells in the spleen and bursa. Cells in the spleen that stain for CD1 are also positive for IgM and consistent with identification of these as B cells. In the skin, chCD1-1 is expressed on cells with dendritic morphology along the dermal-epidermal boundary and in epidermal sheets consistent with chicken Langerhans cells. Staining of cells in the medullary region of the chicken thymus was also observed. The CD1 proteins in mammals traffic to intracellular compartments to acquire lipid antigens for subsequent presentation to T cells on the surface. Consistent with data from mammal CD1 proteins, chCD1-1 partially co-localized with a lysosomal marker in the myeloid cell line BM2. Taken together, these data support broad distribution of chCD1-1 in both lymphoid and non-lymphoid tissues of the chicken that is remarkably similar to the distribution of CD1 isoforms in mammals.

Structural basis for lipid-antigen recognition in avian immunity

CD1 proteins present self- and foreign lipid Ags to activate specific T cells in the mammalian immune system. These T cells play an important role in controlling autoimmune diseases, suppression of tumor growth, and host defense against invading pathogens. Humans use five CD1 isoforms, whereas only two exist in birds. Unlike mammals' CD1, the structure of chicken CD1-2 showed a primitive lipid-binding groove, suggesting that chicken may only recognize single-chain lipids. In contrast, the crystal structure of the second chicken CD1 isoform, chCD1-1, reported in this study at 2.2 A resolution, reveals an elaborated binding groove with a dual-pocket, dual-cleft architecture. The A' and F' deep pockets are separated from each other, but each is connected to a hydrophobic surface cleft, which may participate in lipid binding. The long endogenous ligand found inside the binding groove of chCD1-1, together with binding data on various glycolipids and mycolic acid, strongly suggest that the unique avian CD1 family could bind long dual- and possibly triacyl-chain lipids.

CD1-restricted adaptive immune responses to Mycobacteria in human group 1 CD1 transgenic mice

Group 1 CD1 (CD1a, CD1b, and CD1c)–restricted T cells recognize mycobacterial lipid antigens and are found at higher frequencies in Mycobacterium tuberculosis (Mtb)–infected individuals. However, their role and dynamics during infection remain unknown because of the lack of a suitable small animal model. We have generated human group 1 CD1 transgenic (hCD1Tg) mice that express all three human group 1 CD1 isoforms and support the development of group 1 CD1–restricted T cells with diverse T cell receptor usage. Both mycobacterial infection and immunization with Mtb lipids elicit group 1 CD1–restricted Mtb lipid–specific T cell responses in hCD1Tg mice. In contrast to CD1d-restricted NKT cells, which rapidly respond to initial stimulation but exhibit anergy upon reexposure, group 1 CD1–restricted T cells exhibit delayed primary responses and more rapid secondary responses, similar to conventional T cells. Collectively, our data demonstrate that group 1 CD1–restricted T cells participate in adaptive immune responses upon mycobacterial infection and could serve as targets for the development of novel Mtb vaccines.

A study of mycobacterial lipid antigen-specific responses in a novel model of human group 1 CD1 (45.31)

Group 1 CD1 presents mycobacterial lipid antigens to various T cell subsets. Although group 1 CD1-restricted responses have been implicated in anti-mycobacterial immunity in humans, little is known about their function, as mice lack group 1 CD1. To study group 1 CD1-restricted immune responses in vivo, we have generated human group 1 CD1-transgenic (hCD1Tg) mice. These mice express group 1 CD1 in a pattern similar to humans and support the development of group 1 CD1-restricted T cells, which can be induced both through infection with Mycobacterium tuberculosis (Mtb) and immunization with Mtb lipid antigens. hCD1Tg mice respond to Mtb epitopes recognized by human group 1 CD1-restricted T cells. The kinetics of these group 1 CD1-restricted responses resembles those of conventional T cells and is more rapid upon secondary exposure to antigen, but is distinct from those of group 2 CD1-restricted NKT cells. Furthermore, we find that the number of group 1 CD1-expressing antigen-presenting cells is altered over the course of Mtb infection, and correlates with the kinetics of group 1 CD1-restricted T cells. Taken together, our data demonstrate the viability of hCD1Tg mice as a model for the study of group 1 CD1-restricted T cells in anti-mycobacterial immunity. Such studies could lead to the development of new vaccines against Mtb. This work is supported by NIH grant NIH-A157460.

Serum lipids regulate dendritic cell CD1 expression and function

Dendritic cells (DCs) are highly potent antigen-presenting cells (APCs) and play a vital role in stimulating naïve T cells. Treatment of human blood monocytes with the cytokines granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin (IL)-4 stimulates them to develop into immature dendritic cells (iDCs) in vitro. DCs generated by this pathway have a high capacity to prime and activate resting T cells and prominently express CD1 antigen-presenting molecules on the cell surface. The presence of human serum during the differentiation of iDCs from monocytes inhibits the expression of CD1a, CD1b and CD1c, but not CD1d. Correspondingly, T cells that are restricted by CD1c showed poor responses to DCs that were generated in the presence of human serum, while the responses of CD1d-restricted T cells were enhanced. We chemically fractionated human serum to isolate the bioactive factors that modulate surface expression of CD1 proteins during monocyte to DC differentiation. The human serum components that affected CD1 expression partitioned with polar organic soluble fractions. Lysophosphatidic acid and cardiolipin were identified as lipids present in normal human serum that potently modulate CD1 expression. Control of CD1 expression was mediated at the level of gene transcription and correlated with activation of the peroxisome proliferator-activated receptor (PPAR) nuclear hormone receptors. These findings indicate that the ability of human DCs to present lipid antigens to T cells through expression of CD1 molecules is sensitively regulated by lysophosphatidic acid and cardiolipin in serum, which are ligands that can activate PPAR transcription factors.

Evolutionary biology of CD1

The recognition more than a decade ago that lipids presented by CD1 could function as T cell antigens revealed a startling and previously unappreciated complexity to the adaptive immune system. The initial novelty of lipid antigen presentation by CD1 has since given way to a broader perspective of the immune system's capacity to sense and respond to a diverse array of macromolecules. Some immune recognition systems such as Toll-like receptors can trace their origins back into the deep history of sea urchins and arthropods. Others such as the major histocompatibility complex (MHC) appear relatively recently and interestingly, only in animals that also possess a jaw. The natural history of CD1 is thus part of the wider story of immune system evolution and should be considered in this context. Most evidence indicates that CD1 probably evolved from a classical MHC class I (MHC I) gene at some point during vertebrate evolution. This chapter reviews the evidence for this phylogenetic relationship and attempts to connect CD1 to existing models of MHC evolution. This endeavor is facilitated today by the recent availability of whole genome sequence data from a variety of species. Investigators have used these data to trace the ultimate origin of the MHC to a series of whole genome duplications that occurred roughly 500 million years ago. Sequence data have also revealed homologs of the mammalian MHC I and MHC II gene families in virtually all jawed vertebrates including sharks, bony fishes, reptiles, and birds. In contrast, CD1 genes have thus far been found only in a subset of these animal groups. This pattern of CD1 occurrence in the genomes of living species suggests the emergence of CD 1 in an early terrestrial vertebrate.

BCG vaccine elicits both T-cell mediated and humoral immune responses directed against mycobacterial lipid components

The universe of antigens recognized by alphabeta T cells has recently been expanded to include not only major histocompatibility complex (MHC)-presented protein antigens but also CD1-presented lipid antigens. The significance of lipid-reactive T cells in host defense has been appreciated, using the guinea pig model of human tuberculosis. Here, we show that immunization with Mycobacterium bovis bacillus Calmette-Guerin (BCG), the commonly used anti-tuberculosis vaccine, induces activation of guinea pig cytotoxic T cells recognizing BCG lipids in the context of CD1 molecules. Further, BCG-immunized, but not mock-immunized, guinea pigs mount IgG antibody responses directed against lipoarabinomannnan, an essential cell wall lipid component of mycobacteria. These observations emphasize the ability of BCG to activate the host adaptive immunity to mycobacteria-derived lipids, which could potentially contribute to protection against tuberculosis.

Mycobacterium tuberculosis Regulates CD1 Antigen Presentation Pathways through TLR-2

Mycobacterium tuberculosis remains a major pathogen of worldwide importance, which releases lipid Ags that are presented to human T cells during the course of tuberculosis infections. Here we report that cellular infection with live M. tuberculosis or exposure to mycobacterial cell wall products converted CD1- myeloid precursors into competent APCs that expressed group 1 CD1 proteins (CD1a, CD1b, and CD1c). The appearance of group 1 CD1 proteins at the surface of infected or activated cells occurred via transcriptional regulation, and new CD1 protein synthesis and was accompanied by down-regulation of CD1d transcripts and protein. Isolation of CD1-inducing factors from M. tuberculosis using normal phase chromatography, as well as the use of purified natural and synthetic compounds, showed that this process involved polar lipids that signaled through TLR-2, and we found that TLR-2 was necessary for the up-regulation of CD1 protein expression. Thus, mycobacterial cell wall lipids provide two distinct signals for the activation of lipid-reactive T cells: lipid Ags that activate T cell receptors and lipid adjuvants that activate APCs through TLR-2. These dual activation signals may represent a system for selectively promoting the presentation of exogenous foreign lipids by those myeloid APCs, which come into direct contact with pathogens.

The Mycobacterium tuberculosis serine/threonine kinases PknA and PknB: substrate identification and regulation of cell shape

The Mycobacterium tuberculosis genome contains 11 serine/threonine kinase genes including two, pknA and pknB, that are part of an operon encoding genes involved in cell shape control and cell wall synthesis. Here we demonstrate that pknA and pknB are predominantly expressed during exponential growth, and that overexpression of these kinases slows growth and alters cell morphology. We determined the preferred substrate motifs of PknA and PknB, and identified three in vivo substrates of these kinases: PknB; Wag31, an ortholog of the cell division protein DivIVA; and Rv1422, a conserved protein of unknown function. Expression of different alleles of wag31 in vivo alters cell shape, in a manner dependent on the phosphoacceptor residue in the protein produced. Partial depletion of pknA or pknB results in narrow, elongated cells. These data indicate that signal transduction mediated by these kinases is a novel mechanism for the regulation of cell shape in mycobacteria, one that may be conserved among gram-positive bacteria.

Characterization of two avian MHC-like genes reveals an ancient origin of the CD1 family

Many of the genes that comprise the vertebrate adaptive immune system are conserved across wide evolutionary time scales. Most notably, homologs of the mammalian MHC gene family have been found in virtually all jawed vertebrates, including sharks, bony fishes, reptiles, and birds. The CD1 family of antigen-presenting molecules are related to the MHC class I family but have evolved to bind and present lipid antigens to T cells. Here, we describe two highly divergent nonclassical MHC class I genes found in the chicken (Gallus gallus) that have sequence homology to the mammalian CD1 family of proteins. One of the chicken CD1 genes expresses a full-length transcript, whereas the other has multiple splice variants. Both Southern blot and single nucleotide polymorphism analysis indicates that chicken CD1 is relatively nonpolymorphic. Moreover, cross-hybridizing bands are present in other bird species, suggesting broad conservation in the avian class. Northern analysis of chicken tissue shows a high level of CD1 expression in the bursa and spleen. In addition, molecular modeling predicts that the potential antigen-binding pocket is probably hydrophobic, a universal characteristic of CD1 molecules. Genomic analysis indicates that the CD1 genes are located on chicken chromosome 16 and maps to within 200 kb of the chicken MHC B locus, suggesting that CD1 genes diverged from classical MHC genes while still linked to the major histocompatibility complex locus. The existence of CD1 genes in an avian species suggests that the origin of CD1 extends deep into the evolutionary history of terrestrial vertebrates.

Subcellular localization of mycobacteria in tissues and detection of lipid antigens in organelles using cryo-techniques for light and electron microscopy

The survival of intracellular pathogens within a host is determined by microbial evasion, which can be partially attributed to their subcellular trafficking strategies. Microscopic techniques have become increasingly important in understanding the cell biology of microbial infections. These recently developed techniques can be used for the subcellular localization of antigens not only in cultured cells but also within tissues such as Mycobacterium tuberculosis in lung and Mycobacterium leprae in skin. High-resolution immunofluorescence microscopy can be used in combination with cryo-immunogold electron microscopy using consecutive cryo-sections on the same tissue block forming a direct connection between the two microscopy techniques. The detection of mycobacterial lipid antigens in situ at an ultrastructural level is currently a challenge, but new modifications can be used to address this. These methods might be of interest to microbiologists and cell biologists who study host-pathogen interactions.

Cadherin-11 provides specific cellular adhesion between fibroblast-like synoviocytes

Cadherins are integral membrane proteins expressed in tissue-restricted patterns that mediate homophilic intercellular adhesion. During development, they orchestrate tissue morphogenesis and, in the adult, they determine tissue integrity and architecture. The synovial lining is a condensation of fibroblast-like synoviocytes (FLS) and macrophages one to three cells thick. These cells are embedded within the extracellular matrix, but the structure is neither an epithelium nor an endothelium. Previously, the basis for organization of the synovium into a tissue was unknown. Here, we cloned cadherin-11 from human rheumatoid arthritis (RA)-derived FLS. We developed L cell transfectants expressing cadherin-11, cadherin-11 fusion proteins, and anti–cadherin-11 mAb. Cadherin-11 was found to be expressed mainly in the synovial lining by immunohistologic staining of human synovium. FLS adhered to cadherin-11–Fc, and transfection of cadherin-11 conferred the formation of tissue-like sheets and lining-like structures upon fibroblasts in vitro. These findings support a key role for cadherin-11 in the specific adhesion of FLS and in synovial tissue organization and behavior in health and RA.

Alternative bisphosphonate targets and mechanisms of action

As the number of bisphosphonates continues to increase, they have found widespread use in an increasing number of clinical conditions. Ongoing examination of their targets and mechanisms of action has revealed that this surprisingly diverse class of drugs has effects beyond those first described for osteoclasts. These additional targets include osteoblasts, osteocytes, angiogenesis, and gammadelta T lymphocytes of the human immune system. The immune system effects are specifically targeted to gammadelta T cells and are reminiscent of the effects seen after ingestion of tea beverage. BP effects on such alternate targets may explain not only their antiresorptive effect, but also their effect on bone quality, tumors, and microbes.

Identification of guinea pig gammadelta T cells and characterization during pulmonary tuberculosis

Guinea pigs are an alternative small animal model for many disease studies. Here we describe a pan-gammadelta monoclonal antibody (anti-TCRdelta1) specific for the constant region of human T cell receptor delta chains that cross-reacts with a subpopulation of guinea pig (Cavia porcellus) lymphocytes. The phenotype and distribution of this subpopulation is consistent with the guinea pig gammadelta T cell subset. FACS analysis of fresh PBMC and splenocytes from naïve guinea pigs revealed the presence of a subset of cells that stained with the anti-TCRdelta1 mAb. The relative percentage of anti-TCRdelta1 positive cells in PBMC and tissues is similar to that described for gammadelta T cells in other species. Immunohistochemistry of tissues also revealed a distribution of anti-TCRdelta1 positive cells consistent with gammadelta T cells. These data are further supported by staining of a polyclonal guinea pig T cell line that became progressively CD4 and CD8 negative in long-term culture. Analysis of PBMC from guinea pigs following aerosol infection with virulent Mycobacterium tuberculosis revealed no apparent changes in the steady-state percentage of blood gammadelta+ T cells. Taken together, these data suggest that the anti-TCRdelta1 antibody recognizes the gammadelta T cell subset in guinea pigs. This reagent may be useful for examining gammadelta T cells in various disease models where the guinea pig is a more desirable model for study.

Transcription regulation by the Mycobacterium tuberculosis alternative sigma factor SigD and its role in virulence

Mycobacterium tuberculosis, an obligate mammalian pathogen, adapts to its host during the course of infection via the regulation of gene expression. Of the regulators of transcription that play a role in this response, several alternative sigma factors of M. tuberculosis have been shown to control gene expression in response to stresses, and some of these are required for virulence or persistence in vivo. For this study, we examined the role of the alternative sigma factor SigD in M. tuberculosis gene expression and virulence. Using microarray analysis, we identified several genes whose expression was altered in a strain with a sigD deletion. A small number of these genes, including sigD itself, the gene encoding the autocrine growth factor RpfC, and a gene of unknown function, Rv1815, appear to be directly regulated by this sigma factor. By identifying the in vivo promoters of these genes, we have determined a consensus promoter sequence that is putatively recognized by SigD. The expression of several genes encoding PE-PGRS proteins, part of a large family of related genes of unknown function, was significantly increased in the sigD mutant. We found that the expression of sigD is stable throughout log phase and stationary phase but that it declines rapidly with oxygen depletion. In a mouse infection model, the sigD mutant strain was attenuated, with differences in survival and the inflammatory response in the lung between mice infected with the mutant and those infected with the wild type.

Protection elicited by a double leucine and pantothenate auxotroph of Mycobacterium tuberculosis in guinea pigs

We developed a live, fully attenuated Mycobacterium tuberculosis vaccine candidate strain with two independent attenuating auxotrophic mutations in leucine and pantothenate biosynthesis. The deltaleuD deltapanCD double auxotroph is fully attenuated in the SCID mouse model and highly immunogenic and protective in the extremely sensitive guinea pig tuberculosis model, reducing both bacterial burden and disease pathology.

Epidermal Langerhans cells efficiently mediate CD1a-dependent presentation of microbial lipid antigens to T cells

Langerhans cells are a critical component of skin immunity, capable of capturing protein antigens in the epidermis and presenting them to specific T cells in the context of major histocompatibility complex class II molecules. Recently, a major histocompatibility complex independent pathway of lipid antigen presentation has been identified and is mediated by molecules of the CD1 family (CD1a, CD1b, CD1c, and CD1d). Because Langerhans cells are professional antigen-presenting cells and express CD1a molecules prominently, we hypothesized that Langerhans cells might play a role in T cell responses directed against not only peptide antigens but also lipid antigens. Here, we show that freshly isolated immature Langerhans cells as well as mature Langerhans cells that have migrated from the epidermis are efficient in presenting foreign microbial lipid antigens to specific T cells whereas dermal dendritic cells express much less CD1a molecules and function inefficiently. Further, we found that Langerhans cells migrating from epidermal sheets that were exposed to microbial lipid antigens expressed lipid-antigen-loaded CD1a molecules on the cell surface, resulting in activation of specific T cells. These results underscore an outstanding ability of Langerhans cells to mediate CD1a-dependent lipid antigen presentation. Thus, Langerhans-cell-mediated skin immunity may involve T cell recognition of both peptide and lipid antigens.

Immunization with a mycobacterial lipid vaccine improves pulmonary pathology in the guinea pig model of tuberculosis

Lipids and glycolipid molecules derived from Mycobacterium tuberculosis can be presented to T cells by CD1 cell-surface molecules in humans. These lipid-specific T cells are cytolytic, secrete pro-inflammatory cytokines and have bactericidal activity. Here, we describe studies in which lipids from M. tuberculosis were incorporated into liposomes with adjuvant and tested as vaccines in a guinea pig aerosol tuberculosis challenge model. Animals vaccinated with mycobacterial lipids showed reduced bacterial burdens in the lung and spleen at 4 weeks after infection. In addition, the lungs of lipid-vaccinated animals also had significantly less pathology, with granulomatous lesions being smaller and more lymphocytic. In contrast, animals receiving only vehicle control immunizations had granulomatous lesions that were larger and often contained caseous necrotic centers. Quantification of histopathology by morphometric analysis revealed that the overall percentage of lung occupied by diseased tissue was significantly smaller in lipid-vaccinated animals as compared to vehicle control animals. In addition, the mean area of individual granulomatous lesions was found to be significantly smaller in both lipid- and bacillus Calmette-Guerin-vaccinated guinea pigs. These data support an important role for lipid antigens in the immune response to M. tuberculosis infection, potentially through the generation of CD1-restricted T cells. Immunogenic lipids thus represent a novel class of antigens that might be included to enhance the protective effects of subunit vaccine formulations.

CD1 antigen presentation and infectious disease

Taken together, the data generated thus far strongly suggest that CD1 plays a role in the immune response against various infections (table 1). For obvious reasons, the data gathered thus far using model infection systems have focused primarily on the mouse and therefore only examine the role of CD1d. This leaves an important gap in our understanding of the CD1 antigen presentation pathway given the potential role of CD1a, CD1b and CD1c for contributing to antimicrobial immunity. The functional dichotomy between group 1 and group 2 CD1 isoforms obviously requires further analysis. However, we propose that the group 1 CD1 (CD1a, CD1b, CD1c) antigen presentation pathway is closer to the traditional adaptive immune response mechanisms with the capacity to present unique foreign antigens to specific T cells. This broadens the universe antigens that T cells can use to target pathogens and provides important antimicrobial effector mechanisms that may be critical for combating some types of infections. Lipid antigens may also provide a more effective means of targeting intracellular pathogens by T cells since CD1 is able to sample almost all of the intracellular reservoirs that are exploited by this class of pathogen and may provide an important component of the cytotoxic T cell response [80]. On the other hand, the group 2 CD1 protein (CD1d) may be more intermediate in terms of lying functionally between the innate and adaptive immune systems. The activation of CD1d-restricted T cells may, therefore, help bridge the temporal gap between the onset of innate immunity and the purely adaptive responses typified by the MHC-restricted T cells. Hence, the CD1d-restricted [table: see text] T cells are primed for rapid high-level cytokine release. In addition, the interaction of CD1d-restricted T cells with CD1d on DCs can trigger the release of IL-4 and GM-CSF to promote maturation of tissue-resident DC at the site of infection. The maturation of tissue DC would lead to migration of the activated DC to regional lymph nodes and initiation of MHC-restricted T cell responses. Subsequent IL-12 production by the DC in response to CD1d-mediated T cell stimulation could then drive IFN-gamma production by CD1d-restricted T cells and influence the polarization of the T cell response to infection. In addition, early bursts of IFN-gamma by CD1d-restricted T cells could also upregulate antimicrobial activity in macrophages and activate other important effector cells such as NK cells prior to MHC-restricted T cell responses. In the constant struggle between the microbial pathogen and its host, the evolutionary balance almost always favors the microbe. The rapid rate of evolution and adaptation of the microbe accounts for most of this advantage. Hence, it is not surprising that the host immune system has evolved a complex set of pathways, in addition to the MHC, that are able to recognize and target the unique molecular signatures of infectious microorganisms. The lipid antigens presented by CD1 add to this array and thus provide a further layer of immune defense to the host for combating pathogens.

Characterization of a divergent non-classical MHC class I gene in sharks

Sharks are the most ancient group of vertebrates known to possess members of the major histocompatibility complex (MHC) gene family. For this reason, sharks provide a unique opportunity to gain insight into the evolution of the vertebrate immune system through comparative analysis. Two genes encoding proteins related to the MHC class I gene family were isolated from splenic cDNA derived from spiny dogfish shark ( Squalus acanthias). The genes have been designated MhcSqac-UAA*01 and MhcSqac-UAA*NC1. Comparative analysis demonstrates that the Sqac-UAA*01 protein sequence clusters with classical MHC class I of several shark species and has structural elements common to most classical MHC class I molecules. In contrast, Sqac-UAA*NC1 is highly divergent from all vertebrate classical MHC class I proteins, including the Sqac-UAA *01 sequence and those of other shark species. Although Sqac-UAA*NC1 is clearly related to the MHC class I gene family, no orthologous genes from other species were identified due to the high degree of sequence divergence. In fact, the Sqac NC1 protein sequence is the most divergent MHC class-I-like protein identified thus far in any shark species. This high degree of divergence is similar in magnitude to some of the MHC class-I-related genes found in mammals, such as MICA or CD1. These data support the existence of a class of highly divergent non-classical MHC class I genes in the most primitive vertebrates known to possess homologues of the MHC and other components of the adaptive immune system.

Regulation of tumor necrosis factor alpha gene expression by mycobacteria involves the assembly of a unique enhanceosome dependent on the coactivator proteins CBP/p300

Tumor necrosis factor alpha (TNF-alpha) plays an important role in host containment of infection by Mycobacterium tuberculosis, one of the leading causes of death by an infectious agent globally. Using the pathogenic M. tuberculosis strain H37Rv, we present evidence that upon stimulation of monocytic cells by M. tuberculosis a unique TNF-alpha enhanceosome is formed, and it is distinct from the TNF-alpha enhanceosome that forms in T cells stimulated by antigen engagement or virus infection. A distinct set of activators including ATF-2, c-jun, Ets, Sp1, Egr-1 and the coactivator proteins CBP/p300 are recruited to the TNF-alpha promoter after stimulation with M. tuberculosis. Furthermore, the formation of this enhanceosome is dependent on inducer-specific helical phasing relationships between transcription factor binding sites. We also show that the transcriptional activity of CBP/p300 is potentiated by mycobacterial stimulation of monocytes. The identification of TNF-alpha regulatory elements and coactivators involved in M. tuberculosis-stimulated gene expression thus provides potential selective molecular targets in the modulation of TNF-alpha gene expression in the setting of mycobacterial infection.

Conservation of CD1 intracellular trafficking patterns between mammalian species

Dendritic cells (DC) are potent APCs that sample Ags from the surrounding environment and present them to naive T cells using cell surface Ag-presenting molecules. The DC in both lymphoid and nonlymphoid tissues express high levels of CD1, a cell surface glycoprotein capable of presenting lipids and glycolipids to T cells. Distinct group 1 CD1 isoforms (CD1a, -b, -c) in man are known to traffic to different parts of the endocytic system where microbial Ags may be sampled. Guinea pigs are the only known rodent species that express the group 1 CD1 proteins. Therefore, we examined the expression and trafficking of guinea pig CD1 (gpCD1) isoforms on isolated DC. Confocal microscopy using mAbs specific for individual gpCD1 isoforms revealed differential trafficking of two distinct CD1b isoforms within DC. Colocalization of MHC class II was observed with the gpCD1b1 isoform, consistent with localization in the late endosomes of DC. In contrast, the gpCD1b3 isoform lacks an endosomal sorting motif and remains on the cell surface. Following incubation with Mycobacterium tuberculosis lipoarabinomannan, colocalization of endocytosed lipoarabinomannan with the gpCD1b1 isoform was observed but not with the gpCD1b3 isoform, which remained primarily on the cell surface. These data demonstrate that guinea pig DC express CD1 isoforms with unique trafficking patterns that recapitulate the patterns seen for human CD1 isoforms. This suggests evolutionary pressure for a conserved mechanism in mammals that allows CD1 to sample lipid Ags from various subcompartments of the endocytic system.

Dissemination of Mycobacterium tuberculosis is influenced by host factors and precedes the initiation of T-cell immunity

We report that dissemination of Mycobacterium tuberculosis in the mouse is under host control and precedes the initiation of T-cell immunity. Nine to eleven days after aerosol inoculation, M. tuberculosis disseminates to the pulmonary lymph nodes (LN), where M. tuberculosis-specific T cells are detected 2 to 3 days thereafter. This indicates that the initial spread of bacteria occurs via lymphatic drainage and that the acquired T-cell immune response is generated in the draining LN. Dissemination to peripheral sites, such as the spleen and the liver, occurs 11 to 14 days postinfection and is followed by the appearance of M. tuberculosis-specific T cells in the lung and the spleen. In all cases studied, dissemination to the LN or the spleen preceded activation of M. tuberculosis-specific T cells in that organ. Interestingly, bacteria disseminate earlier from the lungs of resistant C57BL/6 mice than from the lungs of susceptible C3H mice, and consequently, C57BL/6 mice generate an immune response to M. tuberculosis sooner than C3H mice generate an immune response. Thus, instead of spreading infection, early dissemination of M. tuberculosis may aid in the initiation of an appropriate and timely immune response. We hypothesize that this early initiation of immunity following inoculation with M. tuberculosis may contribute to the superior resistance of C57BL/6 mice.

Induction of CD1-restricted immune responses in guinea pigs by immunization with mycobacterial lipid antigens

Group 1 CD1 molecules have been shown to present lipid and glycolipid Ags of mycobacteria to human T cells. However, a suitable animal model for the investigation of this component of antimycobacterial immunity has not yet been established. Previously, we found that guinea pigs express multiple isoforms of group 1 CD1 proteins that are homologous to human CD1b and CD1c. In this study, we show that CD1-restricted T cell responses can be generated in guinea pigs following immunization with lipid Ags from Mycobacterium tuberculosis. Splenic T cells from lipid Ag-immunized guinea pigs showed strong proliferative responses to total lipid Ags and partially purified glycolipid fractions from M. tuberculosis. These lipid Ag-reactive T cells were enriched in CD4-negative T cell fractions and showed cytotoxic activity against CD1-expressing guinea pig bone marrow-derived dendritic cells pulsed with M. tuberculosis lipid Ags. Using guinea pig cell lines transfected with individual CD1 isoforms as target cells in cytotoxic T cell assays, we found that guinea pig CD1b and CD1c molecules presented M. tuberculosis glycolipid Ags to T cells raised by mycobacterial lipid immunization. These results were confirmed using a T cell line derived from M. tuberculosis lipid Ag-immunized guinea pigs, which also showed CD1-restricted responses and cytolytic activity. Our results demonstrate that CD1-restricted responses against microbial glycolipid Ags can be generated in vivo by specific immunization and provide support for the use of the guinea pig as a relevant small animal model for the study of CD1-restricted immune responses to mycobacterial pathogens.

Characterization of guinea-pig group 1 CD1 proteins

CD1 molecules are cell-surface glycoproteins with strong structural similarities to major histocompatibility complex (MHC) class I molecules, and studies in humans and mice have demonstrated that CD1 proteins perform the unique role of presenting lipid antigens to T lymphocytes. Our previous studies have shown that guinea-pigs, unlike the muroid rodents, have an extended family of group 1 CD1 genes. In the current study, we raised monoclonal anibodies (mAbs) against guinea-pig CD1 proteins and generated transfected cell lines expressing individual members of the guinea-pig CD1 family. Our results indicated that multiple members of the guinea-pig CD1 family, including members that are homologous to the human CD1b and CD1c proteins, are expressed at the protein level in transfected cells and in specialized antigen-presenting cells such as monocyte-derived dendritic cells. In addition, CD1 proteins, especially guinea-pig CD1b3, were expressed on a large number of B cells in the guinea-pig, and CD1 expression appeared to be regulated by B-cell maturation or differentiation. Interestingly, three different patterns of intracellular localization were observed for the various guinea-pig CD1 isoforms, a finding that is reminiscent of the distinct patterns of intracellular localization that have been previously demonstrated for human CD1a, CD1b and CD1c. Taken together, these results provide further evidence for substantial similarities between the guinea-pig and human CD1 systems, thus supporting the possibility that the guinea-pig may offer significant advantages as an animal model for the study of the in vivo role of CD1 proteins in infectious and autoimmune diseases.

Extraction of human Langerhans cells: a method for isolation of epidermis-resident dendritic cells

Langerhans cells (LCs) are immature dendritic cells in the epidermis that play a central role in T-lymphocyte mediated skin immunity. Upon activation with antigenic stimuli, they differentiate drastically into mature dendritic cells while migrating from the epidermis to regional lymph nodes. Thus, in order to study biological details of immature LCs, it is crucial to isolate epidermis-resident, immature LCs without dermal dendritic cell contamination. Methods for extracting LCs from human skin as well as in vitro derivation of LC-like cells from hematopoietic progenitor cells have been described previously, but the cell preparations can potentially contain a significant number of dendritic cells that are not identical to epidermal LCs. Here, we describe a technique by which purely epidermis-resident LCs are extracted from human skin. Following digestion of human skin with dispase, the epidermis was separated mechanically without any attached dermal component. The trypsinized epidermal cells were then fractionated by centrifugation with a discontinuous density gradient composed of bovine albumin and sodium metrizoate. The LC-enriched preparation thus obtained contained 80% to >90% CD1a+, E-cadherin+ cells that expressed Birbeck granules and the Lag protein. Consistent with their being at an immature stage, the freshly isolated LCs lacked the expression of CD83, a marker for mature dendritic cells. The purified LCs were able to activate allogeneic T cells, indicating that the cells retained T-cell stimulation ability even after extraction. Thus, the present work offers an opportunity for precise in vitro studies of epidermal LCs.

IL-10-producing T cells suppress immune responses in anergic tuberculosis patients

The lethality of Mycobacterium tuberculosis remains the highest among infectious organisms and is linked to inadequate immune response of the host. Containment and cure of tuberculosis requires an effective cell-mediated immune response, and the absence, during active tuberculosis infection, of delayed-type hypersensitivity (DTH) responses to mycobacterial antigens, defined as anergy, is associated with poor clinical outcome. To investigate the biochemical events associated with this anergy, we screened 206 patients with pulmonary tuberculosis and identified anergic patients by their lack of dermal reactivity to tuberculin purified protein derivative (PPD). In vitro stimulation of T cells with PPD induced production of IL-10, IFN-gamma, and proliferation in PPD(+) patients, whereas cells from anergic patients produced IL-10 but not IFN-gamma and failed to proliferate in response to this treatment. Moreover, in anergic patients IL-10-producing T cells were constitutively present, and T-cell receptor-mediated (TCR-mediated) stimulation resulted in defective phosphorylation of TCRzeta and defective activation of ZAP-70 and MAPK. These results show that T-cell anergy can be induced by antigen in vivo in the intact human host and provide new insights into mechanisms by which M. tuberculosis escapes immune surveillance.

Conservation of a CD1 multigene family in the guinea pig

CD1 is a family of cell-surface molecules capable of presenting microbial lipid Ags to specific T cells. Here we describe the CD1 gene family of the guinea pig (Cavia porcellus). Eight distinct cDNA clones corresponding to CD1 transcripts were isolated from a guinea pig thymocyte cDNA library and completely sequenced. The guinea pig CD1 proteins predicted by translation of the cDNAs included four that can be classified as homologues of human CD1b, three that were homologues of human CD1c, and a single CD1e homologue. These guinea pig CD1 protein sequences contain conserved amino acid residues and hydrophobic domains within the putative Ag binding pocket. A mAb specific for human CD1b cross-reacted with multiple guinea pig CD1 isoforms, thus allowing direct analysis of the structure and expression of at least a subset of guinea pig CD1 proteins. Cell-surface expression of CD1 was detected on cortical thymocytes, dermal dendritic cells in the skin, follicular dendritic cells of lymph nodes, and in the B cell regions within the lymph nodes and spleen. CD1 proteins were also detected on a subset of PBMCs consistent with expression on circulating B cells. This distribution of CD1 staining in guinea pig tissues was thus similar to that seen in other mammals. These data provide the foundation for the development of the guinea pig as an animal model to study the in vivo function of CD1.

Conservation of a CD1 Multigene Family in the Guinea Pig

CD1 is a family of cell-surface molecules capable of presenting microbial lipid Ags to specific T cells. Here we describe the CD1 gene family of the guinea pig (Cavia porcellus). Eight distinct cDNA clones corresponding to CD1 transcripts were isolated from a guinea pig thymocyte cDNA library and completely sequenced. The guinea pig CD1 proteins predicted by translation of the cDNAs included four that can be classified as homologues of human CD1b, three that were homologues of human CD1c, and a single CD1e homologue. These guinea pig CD1 protein sequences contain conserved amino acid residues and hydrophobic domains within the putative Ag binding pocket. A mAb specific for human CD1b cross-reacted with multiple guinea pig CD1 isoforms, thus allowing direct analysis of the structure and expression of at least a subset of guinea pig CD1 proteins. Cell-surface expression of CD1 was detected on cortical thymocytes, dermal dendritic cells in the skin, follicular dendritic cells of lymph nodes, and in the B cell regions within the lymph nodes and spleen. CD1 proteins were also detected on a subset of PBMCs consistent with expression on circulating B cells. This distribution of CD1 staining in guinea pig tissues was thus similar to that seen in other mammals. These data provide the foundation for the development of the guinea pig as an animal model to study the in vivo function of CD1.

Susceptibility of mice deficient in CD1D or TAP1 to infection with Mycobacterium tuberculosis

Cellular immunity against Mycobacterium tuberculosis controls infection in the majority of infected humans. Studies in mice have delineated an important role for CD4(+) T cells and cytokines including interferon gamma and tumor necrosis factor alpha in the response to infection with mycobacteria. Recently, the identification of CD8(+) CD1-restricted T cells that kill M. tuberculosis organisms via granulysin and the rapid death after infection of beta2 microglobulin deficient mice in humans has drawn attention to a critical role for CD8(+) T cells. The nature of mycobacterial-specific CD8(+) T cells has been an enigma because few have been identified in any species. Here, we delineate the contribution of class I MHC-restricted T cells in the defense against tuberculosis as transporter associated with antigen processing (TAP)1-deficient mice died rapidly, bore a greater bacterial burden, and had more severe tissue pathology than control mice. In contrast, CD1D-/- mice were not significantly different in their susceptibility to infection than control mice. This data demonstrates a critical role for TAP-dependent peptide antigen presentation and provides further evidence that class I MHC-restricted CD8(+) T cells, the major T cell subset activated by this antigen processing pathway, play an essential role in immunity to tuberculosis.

CD1-restricted microbial lipid antigen-specific recognition found in the CD8+ alpha beta T cell pool

It is generally accepted that TCR alphabeta+ CD8+ T cells recognize immunogenic peptides bound to MHC-encoded class I molecules. This recognition is a major component of the cellular response mediating immune protection and recovery from viral infections and from certain intracellular bacterial infections. Here, we report two human CD8+ TCR alphabeta+ T cell lines specific for Mycobacterium tuberculosis Ags presented in the context of CD1a or CD1c Ag-presenting molecules. These T cells recognize lipid Ags and display cytotoxicity as well as strong Th cell type I cytokine responses. By extending presentation by the CD1 system to the major TCR alphabeta+ CD8+ T cell pool, this system gains wider applicability beyond the double negative subset of T cells previously shown to have this reactivity. This implies that previous assumptions about the role of CD8+ T cells in microbial immunity may require revision as the relative proportions of CD1-restricted and MHC class I-restricted CD8+ T cells are further defined.

CD1-Restricted Microbial Lipid Antigen-Specific Recognition Found in the CD8+ αβ T Cell Pool

It is generally accepted that TCR αβ+ CD8+ T cells recognize immunogenic peptides bound to MHC-encoded class I molecules. This recognition is a major component of the cellular response mediating immune protection and recovery from viral infections and from certain intracellular bacterial infections. Here, we report two human CD8+ TCR αβ+ T cell lines specific for Mycobacterium tuberculosis Ags presented in the context of CD1a or CD1c Ag-presenting molecules. These T cells recognize lipid Ags and display cytotoxicity as well as strong Th cell type I cytokine responses. By extending presentation by the CD1 system to the major TCR αβ+ CD8+ T cell pool, this system gains wider applicability beyond the double negative subset of T cells previously shown to have this reactivity. This implies that previous assumptions about the role of CD8+ T cells in microbial immunity may require revision as the relative proportions of CD1-restricted and MHC class I-restricted CD8+ T cells are further defined.

Deviation of immune response to Chlamydia psittaci outer membrane protein in lipopolysaccharide-hyporesponsive mice

The outcome of infection is determined by both the quantity and the quality of an induced immune response. In particular, it has been demonstrated for selected pathogens that induction of TH1 or TH2 type helper T-cell subsets determines whether an immune response gives rise to protective immunity or disease-associated immunopathology. The nature of the antigen and the type of antigen-presenting cells recruited in the induction of a response are critical factors that influence the quality of the immune response. Of particular interest in this respect is the immune response to bacterial particles and the impact of cell wall-associated lipopolysaccharide (LPS) on that response. Nonspecific activation of macrophages and B lymphocytes by LPS could skew the phenotype of activated antigen-presenting cells and selectively alter the immunoglobulin isotypes and helper T-cell subsets that are induced following infection. In an initial attempt to detect immune deviation associated with LPS stimulation, we have compared the immunoglobulin isotypes of antibodies specific for the cysteine-rich outer membrane protein Omp2 induced in normal and LPS-hyporesponsive mice following immunization with Chlamydia psittaci strain guinea pig inclusion conjunctivitis whole elementary bodies. We report that there is a dramatic shift of Omp2-specific antibody from predominantly immunoglobulin G2a (IgG2a) isotype in LPS-hyporesponsive mice to high levels of IgG1 isotype in LPS-responder strains. The dependence of the IgG1 isotype shift on the LPS responder status is linked to the structure of the antigen and its natural processing pathway since LPS-hyporesponsive mice are not, in general, deficient in IgG1 antibody production. In particular, the antibody response to purified recombinant Omp2 is predominantly of the IgG1 isotype even in LPS-hyporesponsive mice.

Dissociation of immune determinants of outer membrane proteins of Chlamydia psittaci strain guinea pig inclusion conjunctivitis

Chlamydia trachomatis is an important human pathogen. Research to develop a Chlamydia vaccine has focused on the major outer membrane protein (MOMP). Determinants of this protein elicit serovar-specific neutralizing antibodies which are thought to play a critical role in protective immunity. MOMP-specific antibody responses are highly variable in the polymorphic population. Genetic factors which might influence the MOMP-specific immune response are consequently of particular interest. The C. psittaci strain guinea pig inclusion conjunctivitis (GPIC) is a natural pathogen of the guinea pig that causes both ocular and genital tract infections that closely resemble those caused by C. trachomatis in humans. As such, it provides an excellent model for disease. In this report, we explore the influence of major histocompatibility complex-linked genes on the MOMP-specific antibody response in mice immunized with either whole GPIC elementary bodies or recombinant GPIC MOMP. Our results indicate that the MOMP-specific antibody response is major histocompatibility complex linked such that mice of the H-2d haplotype are high responders while mice of the H-2k haplotype are low responders. We demonstrate that MOMP-specific B cells are present in H-2k strains which are, however, deficient in MOMP-specific helper T cells. Although immunization of low-MOMP-responder strains with whole chlamydial elementary bodies induces high levels of immunoglobulin G antibody specific for Omp2, the cysteine-rich outer membrane protein, MOMP-specific B cells are unable to receive help from Omp2-specific T cells. The failure of intermolecular help from Omp2-specific T cells and related observations raise important issues regarding the processing and presentation of chlamydial antigens and the design of optimal subunit vaccines.

Sequence analysis of a unique temperature phage: mycoplasma virus L2

Mycoplasma virus L2 is a quasi-spherical enveloped virion containing circular double-stranded DNA. L2 infection of Acholeplasma laidlawii host cells leads to a noncytocidal productive infection cycle followed by establishment of lysogeny in all (or most) infected cells, with viral DNA integrated into the host cell genome. The L2 genome has been sequenced and analyzed. L2 DNA is 11,965-bp long and contains 15 open reading frames (ORFs). One of these, ORF13*, has its start codon within and in the same reading frame as ORF13. The ORFs are clustered in four groups separated by noncoding intergenic regions, suggesting that gene expression involves transcription of genes in a cluster into polycistronic mRNA and translation of these genes via translational coupling or reinitiation. Fifteen L2 start codon sites have been defined and resemble those of eubacteria. The N-terminal sequences of two ORFs appear to be signal peptides, and the gene product of one of these may be an L2 virion integral membrane protein. The ORF 5 product has been tentatively identified as an integrase, based on its sequence similarity to site-specific recombinases. The putative attP integration site has been mapped to an intergenic region, 280-bp downstream from ORF 5. Two putative DNA replication ori sites have been mapped. Each is in an intergenic region and contains a DnaA-box bounded by A + T-rich 6-mer repeats.

Heat shock response in mycoplasmas, genome-limited organisms

We have measured the effect of heat shock on three mycoplasmas (Acholeplasma laidlawii K2 and JA1 and Mycoplasma capricolum Kid) and demonstrated the induction of mycoplasma heat shock proteins under these conditions. Increased synthesis of at least 5 heat shock proteins in A. laidlawii K2, 11 heat shock proteins in A. laidlawii JA1, and 7 heat shock proteins in M. capricolum was observed by electrophoretic analysis of proteins from heat-shocked cells in sodium dodecyl sulfate-polyacrylamide gels. In all three strains, major heat shock proteins (66 to 68 and 26 to 29 kilodaltons [kDa]) were found. The 66- to 68-kDa protein cross-reacted with antibody to Escherichia coli DnaK protein, suggesting that this heat shock protein has been conserved in spite of major reductions in genetic complexity during mycoplasma evolution. A. laidlawii also contained a 60-kDa protein that cross-reacted with eubacterial GroEL protein and a 40-kDa protein that cross-reacted with E. coli RecA protein. Unlike with coliphages, the mycoplasma virus L2 progeny yield was not increased when virus was plated on heat-shocked A. laidlawii host cells. However, UV-irradiated L2 virus could be host cell reactivated by both A. laidlawii SOS repair and heat shock systems.