Volume 52 (3): 361-370, 2004 Copyright ©The Histochemical Society, Inc. Immunocytochemical Localization of Histatins in Human Salivary Glands
Department of Diagnostic and Surgical Science, University of Minnesota, School of Dentistry, Minneapolis, Minnesota (MA); Departimento di Citomorfologia, Universita Degli Studi di Cagliari, Cagliari, Italy (MP); Department of Periodontology and Oral Biology, Goldman School of Dental Medicine, Boston University, Boston, Massachusetts (FGO,EJH); and Department of Pediatric Dentistry, School of Dental Medicine, University of Connecticut, Farmington, Connecticut (ARH) Correspondence to: Dr. Arthur R. Hand, Dept. of Pediatric Dentistry, U. of Connecticut Health Center, 263 Farmington Avenue, Farmington, CT 06030-1610. E-mail: hand{at}nso1.uchc.edu
Histatins are a family of salivary proteins with bactericidal and fungicidal activities that contribute to the innate defense of the oral cavity. Histatins are present in the serous granules of the parotid and submandibular glands. The important role of histatins in saliva, and the limited information on their cellular and subcellular distribution, prompted us to further define the localization of histatins in the major salivary glands. Immunogoldsilver staining of 1-µm sections of plastic-embedded tissue with anti-histatin antibody revealed histatin immunoreactivity in the serous acinar cells of the parotid and submandibular glands, the serous demilune cells of the submandibular and sublingual glands, and in occasional intercalated duct cells. No reactivity was seen in mucous cells or in striated or excretory duct cells. Electron microscopic observations of thin sections labeled with anti-histatin and gold-labeled secondary antibodies revealed immunoreactivity associated with the rough endoplasmic reticulum and Golgi complex and in secretory granules of serous acinar and demilune cells. The granules of parotid acinar cells exhibited relatively uniform labeling of their content, whereas the granules of serous cells in the submandibular and sublingual glands showed variable labeling of the dense and light regions of their content. A few intercalated duct cells adjacent to the acinar cells also exhibited labeled granules. These results suggest that the serous cells of the major glands are the main source of histatins in human saliva. They are also consistent with several previous studies demonstrating the variable distribution of different proteins within the granule content. (J Histochem Cytochem 52:361370, 2004)
Key Words: parotid gland submandibular gland sublingual gland serous cells intercalated ducts secretory granules antimicrobial proteins electron microscopy
HUMAN SALIVA contains a large number of proteins and glycoproteins with antimicrobial properties that contribute to the innate defense of the oral cavity. These include lysozyme, lactoferrin, peroxidase, and histatins, among others. Histatins are a family of small, cationic, histidine-rich proteins that have both bactericidal and fungicidal activities (Oppenheim et al. 1986
Histatins are derived from two gene products, histatin 1 and histatin 3. Histatin 1, which is 38 residues in length, is encoded for by the HIS1 gene, whereas histatin 3, consisting of 32 residues, is encoded for by the HIS2 gene (Sabatini and Azen 1989
Several studies have demonstrated significant changes in salivary histatin concentrations under different conditions. In normal healthy individuals, histatin levels decrease with age (Johnson et al. 2000
The main sources of histatins in human saliva are the parotid and submandibular glands. Stimulation of saliva secretion by mechanical or gustatory stimuli results in increased secretion of histatins by both glands (Jensen et al. 1994
Samples of normal parotid, sublingual, and submandibular glands were obtained from consenting male and female patients, aged 4469 years, undergoing surgery at the Otorhinolaryngology Clinic, University of Cagliari, Cagliari, Italy. One sample of normal parotid gland was provided by Drs. Britta and Firoz Rahemtulla (School of Dentistry, University of Alabama, Birmingham, AL). All procedures were approved by the respective institutional committees on human experimentation at the University of Cagliari and University of Alabama.
For light microscopic studies the glands were fixed overnight in either 4% paraformaldehyde or for 14 hr in 3% paraformaldehyde0.1% glutaraldehyde (Polysciences; Warrington, PA) in 0.1 M sodium cacodylate buffer, pH 7.2, then stored in 1% paraformaldehyde in cacodylate buffer at 4C. The samples were rinsed in 0.1 M cacodylate buffer, dehydrated in cold methanol, embedded in LR Gold resin (Polysciences), or Lowicryl K4M resin (Polysciences) and polymerized under UV light (365 nm) at -20C. One-micrometer sections were collected on SuperFrost Plus slides (Fisher Scientific, Pittsburgh, PA) and incubated for 90 min at room temperature (RT) with a goat polyclonal antibody to human histatin (Atkinson et al. 1990
Electron microscope processing employed fixation of small pieces (1 mm3) of tissue from the same patients in either 1% glutaraldehyde or 3% paraformaldehyde0.1% glutaraldehyde in 0.1 M cacodylate buffer for 12 hr, after which the glands were stored at 4C in either 0.1 M cacodylate buffer or 1% paraformaldehyde in cacodylate buffer. After rinsing in buffer the tissues were dehydrated in cold methanol, embedded in either LR Gold or Lowicryl K4M resin, and polymerized as described above. In addition, some of the glutaraldehyde-fixed samples were dehydrated in ethanol, embedded in Polybed epoxy resin (Polysciences), and polymerized at 60C. Ultrathin sections were collected on uncoated or Formvar-coated nickel grids and immunogold labeling was carried out as described previously (Hand 1995
Light Microscopic ImmunogoldSilver Staining Labeling of 1-µm sections of LR Gold- or Lowicryl-embedded salivary glands with the anti-histatin antibody revealed histatin reactivity in serous secretory cells of all three major glands. In the submandibular gland, the supranuclear and apical regions of serous acinar cells were labeled, indicating the presence of histatin in secretory granules (Figures 1A and 1B) . The intensity of the labeling varied from cell to cell. In general, the intercalated ducts were unreactive (Figures 1A and 1B). However, occasional intercalated duct cells adjacent to the serous acini exhibited some labeling over the apical cytoplasm (Figure 1C). No labeling was observed over the cells of the striated and excretory ducts.
The serous acinar cells of the parotid gland also were labeled by the anti-histatin antibody (Figure 1D). The labeling of the acinar cells was more uniform in the parotid than in the submandibular gland. In the sublingual gland, the serous demilune cells were reactive but no labeling was found in the mucous secretory cells (Figure 1E). No labeling or only low levels of nonspecific background were observed when the primary antibody was omitted from the incubation medium or when normal goat serum was substituted for the primary antibody (data not shown). The distribution of histatin labeling in human salivary glands is summarized in Table 1.
Electron Microscopic Immunogold Labeling In sections of serous acinar cells of the submandibular (Figure 2) and parotid (Figure 3) glands and in serous demilune cells of the sublingual gland (Figure 4) , the secretory granules typically exhibited a bi- or tripartite structure and frequently contained a dense core or spherule (Figures 2A, 2B, 3A, 4A, and 4B), as described previously for human salivary gland serous cells (Tandler and Erlandson 1972
Many gold particles were associated with cisternae of rough endoplasmic reticulum (rER) in the serous cells (Figures 2A and 2C). Labeling was present over the lumen of the cisternae and over the cisternal membranes and attached ribosomes. Although relatively few particles were present over the Golgi complex (Figures 2A and 4A), saccules along the trans side of the Golgi complex and small forming secretory granules occasionally were labeled (Figure 4A, inset).
Some intercalated duct cells close to the acini contained apical secretory granules similar to those described by Lantini et al. (1988) The mucous secretory cells of the submandibular and sublingual glands were unlabeled (Figure 4C). No labeling was seen in mitochondria or over nuclei of serous cells. Few gold particles were present in control sections incubated in the absence of the primary antibody (Figure 3C) or with normal goat serum instead of the primary antibody. Occasionally, small nonspecific clusters of gold particles were observed over a few secretory granules, but the majority of granules were free of gold particles. No differences were observed in the cellular distribution of histatin immunoreactivity in sections of tissues embedded in LR Gold, Lowicryl K4M, or Polybed resin. However, labeling of the rER and Golgi complex was very low or undetectable in Polybed sections.
The results of the immunolabeling studies confirm and extend the observations of Takano et al. (1993)
In many species, including humans, the secretory granules of serous salivary gland cells display a variety of substructures (reviewed in Tandler and Phillips 1993
The presence of histatin immunoreactivity in the serous cells of the major salivary glands suggests that these cells are the main source of histatins in saliva. One previous study (Johnson et al. 2000
Histatin immunoreactivity also was found in the granules of some intercalated duct cells. Previous studies have shown that other secretory proteins of serous and mucous cells may be present in intercalated duct cells (Korsrud and Brandtzaeg 1982 The present results indicate that the histatins, like several other salivary proteins involved in innate defense of the oral cavity, are synthesized and secreted predominantly by the serous cells of the major glands. Also like other salivary proteins, they exhibit a variable distribution within the secretory granules. Important remaining questions about the cell biology of histatins include the factors that regulate their synthesis and the site and mechanisms of the proteolytic cleavages that produce the various family members.
Supported in part by a fellowship from the Universita Degli Studi di Cagliari (MP), the University of Connecticut Health Center, and NIH/NIDCR grants DE 05672, DE 07652, and DE 14950 (FGO).
Received for publication August 4, 2003; accepted November 12, 2003
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