- Email: [email protected]
CEA and innate immunity
258
News & Comment
TRENDS in Microbiology Vol.9 No.6 June 2001
Letters
CEA and innate immunity In the March issue of Trends in Microbiology, Sten Hammarström and Vladimir Baranov presented the intriguing hypothesis that the diversity in the human carcinoembryonic antigen (CEA) family might be driven by the gut microflora1. As they alluded to in their article, and as we have reported, several bacterial genera that specifically colonize the respiratory mucosa also target members of this family. In the spirit of this review, I wish to draw attention to two points pertaining to the respiratory environment: the possible outcomes of bacterial interactions with CEA-related cell adhesion molecules (CEACAMs) in health and disease, and the potential of CEACAMs in pathogenassociated molecular pattern (PAMP) recognition. The ligands of Haemophilus influenzae (including typable and non-typable species) and the Neisseriae share a primary binding site at the exposed CFG face of the CEACAM amino-terminal domain. This protein–protein interaction contrasts with the lectin-like interactions associated with gut pathogens. The crucial elements of the adhesiotopes of the Neisseriae and H. influenzae are conserved in the transmembrane as well as the glycosylphosphatidylinositol (GPI)-anchored forms of CEACAMs. Are these molecules present in the respiratory niche favoured by the Neisseriae and H. influenzae, that is, the upper respiratory tract? Several investigators have identified CEA and CEACAM1 molecules on epithelial cells of the tongue, the oesophagus and tonsils2–4. We have confirmed high levels of expression of CEACAMs on the apical surface of the tonsillar epithelium (R. Heyderman and M. Virji, unpublished; Fig.1). Interestingly, recent studies on biopsies collected during tonsillectomy have shown that intracellular meningococci can be found in epithelial cells in individuals free from meningococcal disease5. Thus, in this particular niche, utilization of CEACAMs by microorganisms for adhesion and invasion is a distinct possibility. However, although inflamed tonsillar
tissue expresses high levels of CEACAMs and can concurrently harbour meningococci, it is unknown if this is the case in normal tissues. A low level of expression might lead to surface localization but not invasion. The presence of CEACAM1 isoforms in normal human body fluids including saliva has recently been reported6. Previous studies have shown the presence of CEA in nasal, tracheal and bronchial mucus7–9 and tenfold elevated levels of these molecules were observed in bronchoalveolar lavage fluid from a patient with mucus impaction of the lung10. In a healthy host, the binding of pathogens to mucins or mucus-associated molecules might clear the infection. However, in cystic fibrosis (CF) patients or in patients with chronic obstructive pulmonary disease (COPD), where mucus clearance is impeded, this association might aid in establishment of infection. In this regard, it is interesting that strains of Haemophilus normally resident in the upper respiratory tract are often associated with recurrent lung infections in patients with CF and COPD (Ref. 11). Do CEACAMs have potential as PAMP recognition molecules? Most pathogens express multiple adhesins. Microorganisms that depend on a single
(a)
niche such as the human respiratory tract are particularly adept at genotypic plasticity, generating phenotypic variants at high frequency12; they are also capable of binding to several distinct host molecules individually or simultaneously13. The evolution of such multiplicity or redundancy of adhesins in meningococci and Haemophilus reflects the polymorphic nature of the host immune response, which includes PAMP recognition, as represented by CD14, for example. One can envision microorganismdriven evolution of CEACAMs being akin to the multiple lipopolysaccharide (LPS) recognition systems or C-reactive protein, which rely on specific microbial molecular structures (e.g. lipid A or phosphorylcholine). In the case of meningococcal ligands for CEACAMs, that is, the Opa proteins, a high level of antigenic variation occurs in their surface-exposed loops (a parallel situation might occur for H. influenzae ligands). This is believed to enable meningococci to evade the specific antibody response of the host. The vast majority of Opa variants are nevertheless able to bind to CEACAMs. Is it possible that this very attribute, a molecular feature retained in the face of extensive antigenic variation, is used
(b)
Fig. 1. (a) Low magnification and (b) high magnification immunohistochemical staining of formalin-fixed paraffinembedded tonsillar tissue using the mouse monoclonal antibody T84.1 against human carcinoembryonic antigenrelated cell adhesion molecules (CEACAMs).
http://tim.trends.com 0966-842X/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S0966-842X(01)02020-0
News & Comment
by the host to recognize and dispose of pathogens in the upper respiratory tract, as proposed for the gut. It would be interesting to know more about the patterns of expression of the receptor molecules in the respiratory tract epithelial cells and the identity of the members of the family present in respiratory exudates, and compare these with their counterparts in the gut. With the development of specific probes, such information is becoming available6. In addition, particularly as the mechanisms of targeting of CEACAMs by gut and respiratory pathogens are distinct, it would be very informative to study the patterns of glycosylation of CEACAMs from various sites, as well as analyze the affinity of interactions of pathogens from distinct mucosal niches with the GPIanchored and signalling molecules of this interesting family of surface receptors. Mumtaz Virji Dept of Pathology and Microbiology, University of Bristol, School of Medical Sciences, University Walk, Bristol, UK BS8 1TD. e-mail: [email protected] References 1 Hammarström, S. and Baranov, V. (2001) Is there a role for CEA in innate immunity in the colon? Trends Microbiol. 9, 119–125 2 Hammarström, S. (1999) The carcinoembryonic antigen (CEA) family: structures, suggested functions and expression in normal and malignant tissues. Semin. Cancer Biol. 9, 67–81 3 Bordessoule, D. et al. (1993) Immunohistological patterns of myeloid antigens – tissue distribution of CD13, CD14, CD16, CD31, CD36, CD65, CD66 and CD67. Br. J. Haematol. 83, 370–383 4 Prall, F. et al. (1996) CD66a (BGP), an adhesion molecule of the carcinoembryonic antigen family is expressed in epithelium, endothelium and myeloid cells in a wide range of normal human tissues. J. Histochem. Cytochem. 44, 35–41 5 Sim, C.J. et al. (2000) Underestimation of meningococci in tonsillar tissue by nasopharyngeal swabbing. Lancet 356, 1653–1654 6 Draberova, L. et al. (2000) Soluble isoforms of CEACAM1 containing the A2 domain: increased serum levels in patients with obstructive jaundice and differences in 3 fucosyl-N-acetyl-lactosamine moiety. Immunology 101, 279–287 7 Matsuoka, Y. et al. (1990) Normal bronchial mucus contains high levels of cancer-associated antigens, CA125, CA19-9, and carcinoembryonic antigen. Cancer 65, 506–510 8 Freed, D. and Taylor, G. (1976) Carcinoembryonic antigen and mucus. Br. Med. J. 6013, 836
TRENDS in Microbiology Vol.9 No.6 June 2001
9 Rogalsky, V.Y. (1973) Identity of carcinoembryonal antigen and antigen of mucusproducing cells. Lancet 7815, 1322–1323 10 Matsuo, K. et al. (1997) A patient with bronchial asthma and mucoid impaction who presented with a high concentration of carcinoembryonic antigen in serum. Nihon Kyobu Shikkan Gakkai Zasshi 35, 883–887 11 van Alphen, L. et al. (1995) Virulence factors in the colonization and persistence of bacteria in the airways. Am. J. Respir. Crit. Care Med. 151, 2099–2100 12 Moxon, E.R. et al. (1998) Adaptive evolution of highly mutable loci in pathogenic bacteria. Perspect. Biol. Med. 42, 154–155 13 Virji, M. et al. (1995) Opc- and pilus-dependent interactions of meningococci with human endothelial cells: molecular mechanisms and modulation by surface polysaccharides. Mol. Microbiol. 18, 741–754
O phototroph, o chemotroph, where art thou? In the February issue of Trends in Microbiology, Reysenbach and Cady reviewed the microbiology of ancient and modern hydrothermal systems1. They reviewed recent data, offered some new perspectives and showed the promise and difficulties of interpreting ancient evidence of life from modern analogs. Their review emphasized chemolithoautotrophs but also considered photoautotrophs. I would like to highlight some recent advances relevant to this review on the place of photoautotrophs in relation to chemolithoautotrophs in ancient and modern hydrothermal environments. The earliest rock records of life described by Reysenbach and Cady formed after the period of heavy bombardment. Some of the diverse hydrothermal ecosystems probably supported the early evolution of both thermophilic photolithoautotrophs and chemolithoautotrophs. Distinguishing between the remains of chemotrophs and phototrophs from ancient environments is not easy. Morphological observations contribute compelling evidence for ancient hydrothermal life comparable to life in modern hydrothermal systems. Reysenbach and Cady noted the dominance of preserved filamentous microorganisms in the record. Selective preservation and ease of detection are two biases potentially contributing to this
259
apparent dominance. Even in contemporary microbial mats, large filaments are more easily noticed than smaller, less conspicuous rods and cocci, many of which can only be detected in mat samples by applying a fluorescent stain. Nevertheless, the presence of the filaments is an important and nearly universal characteristic of microbial mats in flowing hydrothermal ecosystems. Filaments form a meshwork that traps sediments and smaller microorganisms, forming complex biofilms that resist washing away. Many filamentous microorganisms in modern hot spring mats glide to escape from depositional burial and/or to reposition themselves appropriately in gradients of life-sustaining chemicals or light2. Evidence for taxis by filamentous microorganisms in ancient biofilms could be a characteristic of either phototrophs or chemotrophs. The colorful phototrophic biofilms of modern hot springs are complex communities of metabolically diverse organisms including chemoorganotrophs and chemolithotrophs3. Attributing chemical signals such as redox changes and isotopic signatures to particular organisms within such a contemporary community is difficult and might be impossible for Precambrian fossils. Reysenbach and Cady noted recent advances in this endeavor. Ion microprobes can target individual microfossils to determine, potentially, the cellular origin of carbon isotopic signals4. Analysis of complex modern hydrothermal communities could clarify the origin of particular Precambrian carbon isotopic signatures5. Both studies distinguish between photosynthetic carbon fractionation using the Calvin cycle in oxygenic cyanobacteria and the 3-hydroxypropionate pathway in the morphologically similar but anoxygenic Chloroflexus. The ion microprobe interpretations4 are now confounded by the fact that a recently described close relative of Chloroflexus, Oscillochloris trichoides, carries out anoxygenic photosynthesis using the Calvin cycle6. Furthermore, although depletion of 13C in 3.5 Ga and older samples is viewed as evidence of autotrophy, it does not allow us to distinguish between relative contributions from photo- and chemolithoautotrophs, particularly in environments where both are found together.
http://tim.trends.com 0966-842X/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S0966-842X(01)02008-X
News & Comment
TRENDS in Microbiology Vol.9 No.6 June 2001
Letters
CEA and innate immunity In the March issue of Trends in Microbiology, Sten Hammarström and Vladimir Baranov presented the intriguing hypothesis that the diversity in the human carcinoembryonic antigen (CEA) family might be driven by the gut microflora1. As they alluded to in their article, and as we have reported, several bacterial genera that specifically colonize the respiratory mucosa also target members of this family. In the spirit of this review, I wish to draw attention to two points pertaining to the respiratory environment: the possible outcomes of bacterial interactions with CEA-related cell adhesion molecules (CEACAMs) in health and disease, and the potential of CEACAMs in pathogenassociated molecular pattern (PAMP) recognition. The ligands of Haemophilus influenzae (including typable and non-typable species) and the Neisseriae share a primary binding site at the exposed CFG face of the CEACAM amino-terminal domain. This protein–protein interaction contrasts with the lectin-like interactions associated with gut pathogens. The crucial elements of the adhesiotopes of the Neisseriae and H. influenzae are conserved in the transmembrane as well as the glycosylphosphatidylinositol (GPI)-anchored forms of CEACAMs. Are these molecules present in the respiratory niche favoured by the Neisseriae and H. influenzae, that is, the upper respiratory tract? Several investigators have identified CEA and CEACAM1 molecules on epithelial cells of the tongue, the oesophagus and tonsils2–4. We have confirmed high levels of expression of CEACAMs on the apical surface of the tonsillar epithelium (R. Heyderman and M. Virji, unpublished; Fig.1). Interestingly, recent studies on biopsies collected during tonsillectomy have shown that intracellular meningococci can be found in epithelial cells in individuals free from meningococcal disease5. Thus, in this particular niche, utilization of CEACAMs by microorganisms for adhesion and invasion is a distinct possibility. However, although inflamed tonsillar
tissue expresses high levels of CEACAMs and can concurrently harbour meningococci, it is unknown if this is the case in normal tissues. A low level of expression might lead to surface localization but not invasion. The presence of CEACAM1 isoforms in normal human body fluids including saliva has recently been reported6. Previous studies have shown the presence of CEA in nasal, tracheal and bronchial mucus7–9 and tenfold elevated levels of these molecules were observed in bronchoalveolar lavage fluid from a patient with mucus impaction of the lung10. In a healthy host, the binding of pathogens to mucins or mucus-associated molecules might clear the infection. However, in cystic fibrosis (CF) patients or in patients with chronic obstructive pulmonary disease (COPD), where mucus clearance is impeded, this association might aid in establishment of infection. In this regard, it is interesting that strains of Haemophilus normally resident in the upper respiratory tract are often associated with recurrent lung infections in patients with CF and COPD (Ref. 11). Do CEACAMs have potential as PAMP recognition molecules? Most pathogens express multiple adhesins. Microorganisms that depend on a single
(a)
niche such as the human respiratory tract are particularly adept at genotypic plasticity, generating phenotypic variants at high frequency12; they are also capable of binding to several distinct host molecules individually or simultaneously13. The evolution of such multiplicity or redundancy of adhesins in meningococci and Haemophilus reflects the polymorphic nature of the host immune response, which includes PAMP recognition, as represented by CD14, for example. One can envision microorganismdriven evolution of CEACAMs being akin to the multiple lipopolysaccharide (LPS) recognition systems or C-reactive protein, which rely on specific microbial molecular structures (e.g. lipid A or phosphorylcholine). In the case of meningococcal ligands for CEACAMs, that is, the Opa proteins, a high level of antigenic variation occurs in their surface-exposed loops (a parallel situation might occur for H. influenzae ligands). This is believed to enable meningococci to evade the specific antibody response of the host. The vast majority of Opa variants are nevertheless able to bind to CEACAMs. Is it possible that this very attribute, a molecular feature retained in the face of extensive antigenic variation, is used
(b)
Fig. 1. (a) Low magnification and (b) high magnification immunohistochemical staining of formalin-fixed paraffinembedded tonsillar tissue using the mouse monoclonal antibody T84.1 against human carcinoembryonic antigenrelated cell adhesion molecules (CEACAMs).
http://tim.trends.com 0966-842X/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S0966-842X(01)02020-0
News & Comment
by the host to recognize and dispose of pathogens in the upper respiratory tract, as proposed for the gut. It would be interesting to know more about the patterns of expression of the receptor molecules in the respiratory tract epithelial cells and the identity of the members of the family present in respiratory exudates, and compare these with their counterparts in the gut. With the development of specific probes, such information is becoming available6. In addition, particularly as the mechanisms of targeting of CEACAMs by gut and respiratory pathogens are distinct, it would be very informative to study the patterns of glycosylation of CEACAMs from various sites, as well as analyze the affinity of interactions of pathogens from distinct mucosal niches with the GPIanchored and signalling molecules of this interesting family of surface receptors. Mumtaz Virji Dept of Pathology and Microbiology, University of Bristol, School of Medical Sciences, University Walk, Bristol, UK BS8 1TD. e-mail: [email protected] References 1 Hammarström, S. and Baranov, V. (2001) Is there a role for CEA in innate immunity in the colon? Trends Microbiol. 9, 119–125 2 Hammarström, S. (1999) The carcinoembryonic antigen (CEA) family: structures, suggested functions and expression in normal and malignant tissues. Semin. Cancer Biol. 9, 67–81 3 Bordessoule, D. et al. (1993) Immunohistological patterns of myeloid antigens – tissue distribution of CD13, CD14, CD16, CD31, CD36, CD65, CD66 and CD67. Br. J. Haematol. 83, 370–383 4 Prall, F. et al. (1996) CD66a (BGP), an adhesion molecule of the carcinoembryonic antigen family is expressed in epithelium, endothelium and myeloid cells in a wide range of normal human tissues. J. Histochem. Cytochem. 44, 35–41 5 Sim, C.J. et al. (2000) Underestimation of meningococci in tonsillar tissue by nasopharyngeal swabbing. Lancet 356, 1653–1654 6 Draberova, L. et al. (2000) Soluble isoforms of CEACAM1 containing the A2 domain: increased serum levels in patients with obstructive jaundice and differences in 3 fucosyl-N-acetyl-lactosamine moiety. Immunology 101, 279–287 7 Matsuoka, Y. et al. (1990) Normal bronchial mucus contains high levels of cancer-associated antigens, CA125, CA19-9, and carcinoembryonic antigen. Cancer 65, 506–510 8 Freed, D. and Taylor, G. (1976) Carcinoembryonic antigen and mucus. Br. Med. J. 6013, 836
TRENDS in Microbiology Vol.9 No.6 June 2001
9 Rogalsky, V.Y. (1973) Identity of carcinoembryonal antigen and antigen of mucusproducing cells. Lancet 7815, 1322–1323 10 Matsuo, K. et al. (1997) A patient with bronchial asthma and mucoid impaction who presented with a high concentration of carcinoembryonic antigen in serum. Nihon Kyobu Shikkan Gakkai Zasshi 35, 883–887 11 van Alphen, L. et al. (1995) Virulence factors in the colonization and persistence of bacteria in the airways. Am. J. Respir. Crit. Care Med. 151, 2099–2100 12 Moxon, E.R. et al. (1998) Adaptive evolution of highly mutable loci in pathogenic bacteria. Perspect. Biol. Med. 42, 154–155 13 Virji, M. et al. (1995) Opc- and pilus-dependent interactions of meningococci with human endothelial cells: molecular mechanisms and modulation by surface polysaccharides. Mol. Microbiol. 18, 741–754
O phototroph, o chemotroph, where art thou? In the February issue of Trends in Microbiology, Reysenbach and Cady reviewed the microbiology of ancient and modern hydrothermal systems1. They reviewed recent data, offered some new perspectives and showed the promise and difficulties of interpreting ancient evidence of life from modern analogs. Their review emphasized chemolithoautotrophs but also considered photoautotrophs. I would like to highlight some recent advances relevant to this review on the place of photoautotrophs in relation to chemolithoautotrophs in ancient and modern hydrothermal environments. The earliest rock records of life described by Reysenbach and Cady formed after the period of heavy bombardment. Some of the diverse hydrothermal ecosystems probably supported the early evolution of both thermophilic photolithoautotrophs and chemolithoautotrophs. Distinguishing between the remains of chemotrophs and phototrophs from ancient environments is not easy. Morphological observations contribute compelling evidence for ancient hydrothermal life comparable to life in modern hydrothermal systems. Reysenbach and Cady noted the dominance of preserved filamentous microorganisms in the record. Selective preservation and ease of detection are two biases potentially contributing to this
259
apparent dominance. Even in contemporary microbial mats, large filaments are more easily noticed than smaller, less conspicuous rods and cocci, many of which can only be detected in mat samples by applying a fluorescent stain. Nevertheless, the presence of the filaments is an important and nearly universal characteristic of microbial mats in flowing hydrothermal ecosystems. Filaments form a meshwork that traps sediments and smaller microorganisms, forming complex biofilms that resist washing away. Many filamentous microorganisms in modern hot spring mats glide to escape from depositional burial and/or to reposition themselves appropriately in gradients of life-sustaining chemicals or light2. Evidence for taxis by filamentous microorganisms in ancient biofilms could be a characteristic of either phototrophs or chemotrophs. The colorful phototrophic biofilms of modern hot springs are complex communities of metabolically diverse organisms including chemoorganotrophs and chemolithotrophs3. Attributing chemical signals such as redox changes and isotopic signatures to particular organisms within such a contemporary community is difficult and might be impossible for Precambrian fossils. Reysenbach and Cady noted recent advances in this endeavor. Ion microprobes can target individual microfossils to determine, potentially, the cellular origin of carbon isotopic signals4. Analysis of complex modern hydrothermal communities could clarify the origin of particular Precambrian carbon isotopic signatures5. Both studies distinguish between photosynthetic carbon fractionation using the Calvin cycle in oxygenic cyanobacteria and the 3-hydroxypropionate pathway in the morphologically similar but anoxygenic Chloroflexus. The ion microprobe interpretations4 are now confounded by the fact that a recently described close relative of Chloroflexus, Oscillochloris trichoides, carries out anoxygenic photosynthesis using the Calvin cycle6. Furthermore, although depletion of 13C in 3.5 Ga and older samples is viewed as evidence of autotrophy, it does not allow us to distinguish between relative contributions from photo- and chemolithoautotrophs, particularly in environments where both are found together.
http://tim.trends.com 0966-842X/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S0966-842X(01)02008-X