The Epithelial Cell Glycocalyx in Ocular Surface Infection.

The glycocalyx is the main component of the transcellular barrier located at the interface between the ocular surface epithelia and the external environment. This barrier extends up to 500 nm from the plasma membrane and projects into the tear fluid bathing the surface of the eye. Under homeostatic conditions, defense molecules in the glycocalyx, such as transmembrane mucins, resist infection. However, many pathogenic microorganisms have evolved to exploit components of the glycocalyx in order to gain access to epithelial cells and consequently exert deleterious effects. This manuscript reviews the implications of the ocular surface epithelial glycocalyx to bacterial, viral, fungal and parasitic infection. Moreover, it presents some ongoing controversies surrounding the functional relevance of the epithelial glycocalyx to ocular infectious disease.

The cornea IV immunology, infection, neovascularization, and surgery chapter 1: Corneal immunology.

PURPOSE: of Review: This review offers an informed and up-to-date insight on the immune profile of the cornea and the factors that govern the regulation of such a unique immune environment. SUMMARY: The cornea is a unique tissue that performs the specialized task of allowing light to penetrate for visual interpretation. To accomplish this, the ocular surface requires a distinct immune environment that is achieved through unique structural, cellular and molecular factors. Not only must the cornea be able to fend off invasive infectious agents but also control the inflammatory response as to avoid collateral, and potentially blinding damage; particularly of post-mitotic cells such as the corneal endothelium. To combat infections, both innate and adaptive arms of the inflammatory immune response are at play in the cornea. Dendritic cells play a critical role in coordinating both these responses in order to fend off infections. On the other side of the spectrum, the ocular surface is also endowed with a variety of anatomic and physiologic components that aid in regulating the immune response to prevent excessive, potentially damaging, inflammation. This attenuation of the immune response is termed immune privilege. The balance between pro and anti-inflammatory reactions is key for preservation of the functional integrity of the cornea. RECENT FINDINGS: The understanding of the molecular and cellular factors governing corneal immunology and its response to antigens is a growing field. Dendritic cells in the normal cornea play a crucial role in combating infections and coordinating the inflammatory arms of the immune response, particularly through coordination with T-helper cells. The role of neuropeptides is recently becoming more highlighted with different factors working on both sides of the inflammatory balance.

Immune checkpoint inhibitors: what neuro-ophthalmologists need to know.

PURPOSE OF REVIEW: Immune checkpoint inhibitors are currently an exceedingly powerful tool in the management of hitherto incurable malignancies and their use in clinical practice is expected to increase in the near future. The purpose of this review is to discuss the current medical uses of checkpoint inhibitors with a focus on their neuro-ophthalmic side-effects. RECENT FINDINGS: Immune checkpoint inhibitors have emerged as a promising breakthrough in the treatment of several tumor types. However, these targeted therapies can induce a wide range of immune-related ophthalmic and neuro-ophthalmic toxicities. It is important for neuro-ophthamologists to promptly recognize and manage these adverse events that can potentially threaten vision. SUMMARY: There are currently seven FDA-approved immune checkpoint inhibitors and several ones are under investigation. In general, immunotherapy is considered a well tolerated, safe and efficacious treatment option for many cancer patients. Nevertheless, because of their unique mechanism of action, these molecules can alter the immune response and result in immune-related adverse effects in almost every organ with an estimated incidence of ophthalmic side effects in this patient population of less than 1%.

TIPE2 Suppresses Pseudomonas aeruginosa Keratitis by Inhibiting NF-κB Signaling and the Infiltration of Inflammatory Cells.

BACKGROUND: The role of tumor necrosis factor α (TNF-α) induced protein 8-like-2 (TIPE2) in Pseudomonas aeruginosa (PA) keratitis was explored. METHODS: Eight-week-old TIPE2 knockout (TIPE2-/-) C57BL/6 mice and their wild-type (WT) littermates were used. Corneal disease was graded at 1, 2, and 3 days postinfection, and slit lamp, clinical score, histopathology, and immunostaining were performed in the infected corneas. The corneas were harvested, and messenger ribonucleic acid (mRNA) levels of TNF-α, interleukin-1ß (IL-1ß), and interleukin-6 (IL-6) were tested. Enzyme-linked immunosorbent assay (ELISA) determined the protein levels, and nuclear factor κ-light-chain-enhancer of activated B cell (NF-κB) signaling molecules were tested by Western blot. In vitro human corneal epithelial cells (HCECs) were used to determine the relationship between TIPE2 and TAK1. The HCECs were treated with TIPE2 short hairpin ribonucleic acid (shRNA) and lipopolysaccharide (LPS) to test the NF-κB signaling molecules by Western blot. RESULTS: Pseudomonas aeruginosa infection induced a decreased expression of TIPE2 in mouse corneas 2 days postinfection. Compared with the control group, TIPE2-deficient mice were susceptible to infection with PA and showed increased corneal inflammation. Reduced NF-κB signaling and inflammatory cell infiltration were required in the TIPE2-mediated immune modulation. CONCLUSIONS: TIPE2 promoted host resistance to PA infection by suppressing corneal inflammation via regulating TAK1 signaling negatively and inhibiting the infiltration of inflammatory cells.

Uveitis causes according to immune status of patients.

PURPOSE: The advances in medicine have led to an increased number of people living with some form of immunodeficiency. Most ocular infections in immunocompromised patients may lead to irreversible blindness. We identify the causes of uveitis in immunocompetent and immunocompromised patients. METHODS: A retrospective cohort study of 1354 consecutive patients. All patients underwent a standard work-up for uveitis. RESULTS: An immunocompromised state was identified in 171/1354 patients (13%), of whom 40 had Human immunodeficiency virus (HIV) infection, 52 received immunosuppressive medications, 28 had concurrent malignant disorder and 20 had other causes for their immunosuppression. In addition, 93/1354 patients (7%) had diabetes mellitus (DM). The prevalence of intraocular infections was much higher in immunocompromised patients than in immunocompetent patients and DM (p < 0.001). Causes of uveitis differed between the diverse immunocompromised groups. The non-HIV immunocompromised patients showed primarily intraocular herpes simplex and varicella zoster virus infections, whilst HIV-positive patients exhibited frequently cytomegalovirus (CMV) retinitis and syphilis. Patients with generalized malignancies were characterized by a lower prevalence of infections and higher prevalence of sarcoidosis. Patients with DM typically showed sarcoidosis and bacterial intraocular infections. The percentage of undetermined uveitis diagnoses was markedly lower in immunosuppressed patients (p < 0.001). CONCLUSION: In immunocompromised patients with uveitis, infections were diagnosed in 46% of cases in contrast to 12% in the immunocompetent patients. The causes of uveitis differed among the various types of immunosuppression. Immunocompromised patients with uveitis require a rapid assessment for the most expected infections.

STAT-3-independent production of IL-17 by mouse innate-like αß T cells controls ocular infection.

Appropriate regulation of IL-17 production in the host can mean the difference between effective control of pathogens and uncontrolled inflammation that causes tissue damage. Investigation of conventional CD4+ T cells (Th17 cells) has yielded invaluable insights into IL-17 function and its regulation. More recently, we and others reported production of IL-17 from innate αß+ T cell populations, which was shown to occur primarily via IL-23R signaling through the transcription factor STAT-3. In our current study, we identify promyelocytic leukemia zinc finger (PLZF)-expressing iNKT, CD4-/CD8+, and CD4-/CD8- (DN) αß+T cells, which produce IL-17 in response to TCR and IL-1 receptor ligation independently of STAT-3 signaling. Notably, this noncanonical pathway of IL-17 production may be important in mucosal defense and is by itself sufficient to control pathogenic Staphylococcus aureus infection at the ocular surface.

Human antimicrobial peptides in ocular surface defense.

Sight depends on the passage of light through the transparent cornea and being focused on the fovea. Its exposed position renders it vulnerable to microbial infection. The cornea has developed a wide array of defense mechanisms against infection, of which endogenous antimicrobial peptides (AMPs) are key. AMPs are essentially small molecular weight cationic peptides with a wide range of activity against virus, bacteria, fungi and parasites. Some proteins such as RNases and S100As are also included in this group. Several AMPs act synergistically allowing low expression of multiple AMPs to act efficiently. AMPs also have a range of non-microbicidal functions and serve as signaling molecules, immunomodulators; show anti-tumour activity, and influence vascularization and wound healing. Different toll-like receptors (TLR) have been implicated in the preferential induction of specific AMPs. A range of bacteria, including mycobacteria tuberculosis, viruses including herpes virus, fungi and parasites including acanthamoeba, that cause ocular infections have been shown to induce specific AMPs via TLR activation. Non-TLR mediated induction of AMP expression can occur and several molecules such as L-isoleucine, sodium butyrate, vitamin D3, phenylbutyrate, vasoactive intestinal peptide, and etinostat have been identified in this regard. Given the rising microbe resistance to antibiotics, the slow rate of development of new antibiotics and the limited access to effective antibiotics by patients living in the developing world, an ideal solution would be to find AMPs that are effective singly or in combination with each other or other antimicrobial proteins to reduce, if possible eliminate reliance on antibiotics alone.

A Mouse Model for Ocular Surface Staphylococcus aureus Infection.

Creation of an appropriate animal model that accurately reflects the disease and host immune response to bacterial infection in humans is a major challenge in ocular-surface infection research. For decades, mice have been the ideal small animal model for ocular-surface infection research because of the availability and relatively low cost of various genetic backgrounds, targeted defects, and immunologic reagents. By employing different combinations of mouse and bacterial strains, murine infection models can be used to explore a complete picture of bacterial infection and innate immunity of the ocular surface. A murine model of Staphylococcus aureus infection under normal ocular circumstances is presented here as a convenient and tractable model system in which to study mammalian host responses to pathogens. © 2017 by John Wiley & Sons, Inc.

Roles of IL-8 in ocular inflammations: a review.

INTRODUCTION: This review presents the current in vitro and in vivo animal and human research on the roles of IL-8 in ocular inflammatory diseases. MATERIALS AND METHODS: Data sources were a literature review using Pub Med, Medline, and ISI databases (from 1990 to 2011). Search items included interleukine-8 (IL-8), CXCL8, chemokines, cytokines, alone or in combination with the, serum, aqueous, vitreous, eye, ocular, ocular tissues, ophthalmic, and review. RESULTS: IL-8 may be involved in primary or secondary ocular inflammations. Ocular effects of IL-8 differ based on the source of the secretion and site of the action. The most important effects of IL-8 in the eyes are angiogenic activities and induction of ocular inflammation. CONCLUSION: IL-8 plays important roles in ocular inflammation and angiogenesis in conjunctiva, cornea, iris, retina, and orbit. Anti-IL-8 targeted immunotherapy has been introduced as an important treatment modality, provided that IL-8 signal blocking takes place in desired areas and tissues.

Ocular infection of mice with an avirulent recombinant HSV-1 expressing IL-4 and an attenuated HSV-1 strain generates virulent recombinants in vivo.

PURPOSE: To assess the relative impact of overexpression of interleukin 2 (IL-2), interleukin 4 (IL-4), and interferon gamma (IFN-γ) expressing recombinant herpes simplex virus type 1 (HSV-1) on altering immune responses in ocularly infected mice. METHODS: BALB/c mice were co-infected ocularly with avirulent HSV-1 strain KOS and avirulent recombinant HSV-1 expressing murine IL-4 (HSV-IL-4). Controls mice were co-infected with KOS+HSV-IL-2 or KOS+HSV-IFNγ. Following ocular infection, virus replication in the eye, corneal scarring (CS), and survival were determined. We also isolated recombinant viruses from eye and trigeminal ganglia of KOS+HSV-IL-4 infected mice. RESULTS: In this study we found that ocular infection of BALB/c mice with a mixture of HSV-IL-4 and KOS resulted in increased death and increased eye disease. In contrast, when mice were infected in one eye with KOS and the other eye with HSV-IL-4 no death or eye disease was seen. Intraperitoneal co-infection of mice with KOS and HSV-IL-4 also did not result in HSV-1 induced death. Interestingly, ocular infection of mice with a mixture of HSV-IL-2 and KOS did not have any effect on severity of the disease in infected mice. We isolated recombinant viruses from KOS+HSV-IL-4 infected mice eye and trigeminal ganglia. Some of the isolated viruses were more neurovirulent then either parental virus. Infection of macrophages with IL-4 expressing virus down-regulated IL-12 production by macrophages. CONCLUSIONS: These results suggest a role for IL-4 in suppression of immune response and generation of virulent viruses in vivo.

Intravitreal infliximab for the treatment of sight-threatening chronic noninfectious uveitis.

PURPOSE: Tumor necrosis factor-alpha is known to play an important role in various immune-mediated ocular diseases. Infliximab, a chimeric monoclonal antibody against tumor necrosis factor-alpha, has been used for the treatment of various chronic systemic and ocular inflammatory diseases. The purpose of this study was to evaluate the effect of intravitreal injection of infliximab on the visual acuity and central macular thickness in patients with chronic noninfectious uveitis. METHODS: Ten eyes of 7 patients with chronic persistent noninfectious uveitis who were nonresponsive to conventional previous medications during the previous 3 months were included in this study. The patients received intravitreal injection of 1.5 mg/0.15 mL infliximab. Mean best-corrected visual acuity and mean central macular thickness 1 day before and 4 weeks after the injection were evaluated and compared. RESULTS: Mean logarithm of the minimum angle of resolution before and after injection was 1.37 ± 0.43 and 0.67 ± 0.56, respectively, with statistically significant improvement after injection (P = 0.005). The mean central macular thickness before and after injection was 673.20 ± 338.39 µm and 456.40 ± 317.46 µm, respectively, with a significant decrease in the central macular thickness after the injection (P = 0.005). CONCLUSION: Intravitreal injection of infliximab may improve the visual acuity and decrease the central macular thickness in patients with chronic noninfectious uveitis and significant visual loss and central macular edema.

The use of infliximab in ocular inflammation.

AIMS: Experience with anti-tumour necrosis factor alpha medications in ophthalmology has been mainly in the treatment of resistant uveitis and scleritis. There have been a few case reports and one case series detailing the use of infliximab in the treatment of orbital inflammatory disease, but there is still limited experience with these agents in a variety of orbital inflammatory conditions. We describe successful suppression of inflammation with infliximab in the Idiopathic orbital inflammation, thyroid associated orbital inflammation and chronic relapsing inflammatory optic neuropathy. METHODS: This is a retrospective observational case series of five cases of orbital and ocular inflammation. RESULTS: We describe effects of infliximab in the treatment of three cases of orbital inflammation and two cases of chronic relapsing inflammatory optic neuropathy, all of which were unresponsive to alternative immune modulators. The patients received relief of symptoms and signs of inflammation in all five cases. However, this relief was not absolute as symptoms returned around week 5 post-inflammation. CONCLUSION: This report highlights the expanding role of the biological agents in ophthalmology. Although not the answer in treatment, they offer a valuable alternative to steroids in cases of refractory inflammation.

Circulating anti-galectin-1 antibodies are associated with the severity of ocular disease in autoimmune and infectious uveitis.

PURPOSE: Galectin (Gal)-1, an endogenous lectin found at sites of immune privilege, plays a critical role in the regulation of the immune response. Therapeutic administration of Gal-1 or its genetic delivery suppresses chronic inflammation in experimental models of autoimmunity. The purpose of this work was to investigate the occurrence of circulating anti-Gal-1 antibodies in patients with autoimmune and infectious uveitis as potential determinant factors of disease progression. METHODS: IgG, IgE, and IgA anti-Gal-1 antibodies were assessed by ELISA and Western blot in sera from patients with autoimmune (n = 47) and infectious (n = 15) uveitis compared with healthy control subjects (n = 30). The frequency of anti-Gal-1 antibodies was examined in patients experiencing poor clinical outcome (n = 21) or good evolution (n = 9). Anti-Gal-1 antibodies were eluted by incubating patient sera with nitrocellulose filters adsorbed with rGal-1. The ability of these antibodies to recognize retinal tissue was assessed by ELISA, Western blot, and immunohistochemistry. RESULTS: IgE, IgG, and IgA anti-Gal-1 antibodies were increased in sera from patients with autoimmune uveitis (P < 0.001 vs. controls) and toxoplasmic retinochoroiditis (P < 0.001). The level of anti-Gal-1 IgE and IgG antibodies was associated with progressive disease and poor outcome in autoimmune and infectious uveitis. Furthermore, these antibodies strongly immunoreacted with retinal lysates and recognized retinal structures mainly photoreceptors in retinal sections. CONCLUSIONS: Anti-retinal Gal-1 antibodies are present in sera from patients with uveitis and can be associated with the progression of ocular disease, suggesting their potential use in follow-up observations of these patients.

Consequences of CXCL10 and IL-6 induction by the murine IFN-alpha1 transgene in ocular herpes simplex virus type 1 infection.

Herpes simplex virus type 1 infection of the mouse eye results in an impressive inflammatory response culminating in the death of the animal or the establishment of a "latent" infection depending on a number of ill-defined variables that include components of the innate and adaptive immune system. The application of type I interferon transgenes has been found to antagonize viral replication and spread from the eye to the nervous system. Associated with the in situ transfection of the cornea is the upregulation of two inflammatory molecules, interleukin-6 and CXCL10. In this article, we will further examine the contribution these molecules may have in the host response to ocular infection with herpes simplex virus type 1.

No apparent association between periocular and ocular microcolonization and the degree of inflammation in patients with atopic keratoconjunctivitis.

BACKGROUND: The cause of the chronic inflammation in atopic keratoconjunctivitis (AKC), the ocular manifestation of atopic eczema/dermatitis syndrome, is largely unknown. OBJECTIVE: To investigate the possibility that microorganisms may be important in the inflammatory activity in AKC. METHODS: Fifteen patients with AKC participated in the study. The presence of aerobic bacteria and fungi was related to the severity of clinical signs, the numbers of inflammatory cells in tears and conjunctival biopsies, and the concentration of various cytokines in tears. In addition, serological evidence for IgE sensitization to Staphylococcus aureus B antigen and Malassezia sympodialis antigen was investigated. Twelve healthy subjects were included for control purposes. RESULTS: The patients exhibited moderate clinical signs of AKC. No relation was found between the severity of AKC and the presence of microorganisms, despite the fact that S. aureus was frequently isolated. AKC patients showed significantly higher levels of IFN-gamma, TNF-alpha (tumour necrosis factor-alpha), IL-2, IL-4, IL-5 and IL-10 than controls. An association was found between conjunctival signs and the levels of all cytokines except IL-5. CONCLUSION: We found no evidence to suggest that periocular and ocular microcolonization are related to inflammatory parameters in AKC. However, confirmation of the present results in a longitudinal study with repeated clinical examinations and samplings in the same individual is required before the contribution of S. aureus to on-going inflammation in AKC can be dismissed.

[Ocular infections of the elderly]. / Les infections oculaires du sujet âgé.

A CLINICAL ASPECT DEPENDING ON THE PHYSIOPATHOGENESIS: Ocular infections are a frequent motive for ophthalmological consultations in geriatric settings because of the mechanical factors related to age (modifications in palpebral dynamics and lacrymal function) and in local and general immune factors leading to the rapid and/or more severe development of infections. The mechanism of microbial contamination of the eye also determines the clinical damage: predominantly local (dirty hands, traumas) with involvement of the surface tissues (conjunctive and cornea) or general, hematogenic or neurogenic, frequently at the origin of more internal infections (iris, choroid, retina, optical nerve). CONJUNCTIVITIS AND KERATITIS: These provoke reddening of the eyes, tears and above all pain when the corneal epithelium is involved. Microbiological samples are useful in cases of severe, presumably infectious keratitis or conjunctivitis. Two emergency situations must be distinguished: any suspicion of herpes for which local corticosteroids are contraindicated and keratitis or conjunctivitis with the use of lenses, often due to Gram negative bacilli, amoeba or fungus, the treatment of which is intensive and the prognosis often severe. OPHTHALMOLOGICAL HERPES ZOSTER: The rapid diagnosis and introduction of efficient doses of antivirals reduces the initial pain, the ocular complications of herpes zoster and post-zoster pain. The latter, when it exists, requires specialized management. ACUTE UVEITIS: A context of intra-ocular inflammation in an elderly patient must always evoke a pseudo-uveitis syndrome, the principle cause of which is lymphoma. Conversely, an uveitis occurring in the days or weeks following ocular surgery, including cataract, must be considered as suggestive of a post-surgical infection and rapidly referred to a specialist. ACUTE DACRYOCYSTITIS: Is manifested by a hard and painful tumefaction below the internal angle of the eye. Following collection, it requires draining through an in incision in the skin, washing and packing of the sac, and systemic antibiotherapy. The preventive treatment of recurrences requires open dacryocystorhinostomy or via endonasal endoscopy.

Ocular immune privilege and the impact of intraocular inflammation.

Immune privilege, a characteristic of the internal compartments of the eye, is a physiologic mechanism that is designed to provide the eye with protection against pathogens while protecting the delicate visual axis from the sight-destroying potential of immunogenic inflammation. It is assumed that the presence of intraocular inflammation is incompatible with the existence of immune privilege. The validity of this assumption has been tested in four animal models of intraocular inflammation-systemic and local endotoxin-induced uveitis (EIU), mycobacterial adjuvant-induced uveitis (MAIU), and experimental autoimmune uveitis (EAU). Immune privilege was assessed in inflamed eyes by growth of intracamerally injected allogeneic tumor cells, by the capacity to support immune deviation following intracameral injection of antigen (ovalbumin, OVA), by assaying protein, leukocyte, and selected cytokine content of aqueous humor (AqH), and by capacity of inflamed AqH to suppress T cell activation in vitro. The results indicate that, irrespective of the type of inflammation, tumor cells formed progressively growing tumors in inflamed eyes. Moreover, OVA injected into the anterior chamber of eyes inflamed by MAIU and EAU failed to induce immune deviation. AqH from inflamed eyes reflected breakdown of the blood:ocular barrier as well as transient loss of its immunosuppressive properties. Immunosuppressive microenvironments routinely reemerged in inflamed eyes, and the immunosuppressive agent present under these circumstances in AqH was active TGF beta2. It is concluded that immune privilege is surprisingly resistant to abolition by intraocular inflammation, and that maintenance of immune privilege in the face of ongoing inflammation depends upon the emergence of progressive and partially different immunosuppressive mechanisms.

Twenty-five-year panorama of corneal immunology: emerging concepts in the immunopathogenesis of microbial keratitis, peripheral ulcerative keratitis, and corneal transplant rejection.

PURPOSE: To describe the most recent advances in our understanding of the cellular and molecular mechanisms involved in the immunopathogenesis of corneal immunoinflammatory disorders including microbial keratitis, peripheral ulcerative keratitis. and allograft rejection. METHODS: Review of the published peer-reviewed literature that has contributed significantly to our modern understanding of corneal immunology. In addition, the authors have summarized the information in conceptual diagrams that highlight the critical cellular and molecular pathways that lead to corneal immune responses in the two most thoroughly studied corneal immune disorders, herpes simplex keratitis (HSK) and transplant rejection. RESULTS: In spite of the wide array of molecular and cellular factors that mediate corneal immunity, critical mechanistic facets are shared by the various corneal immunoinflammatory disorders. These include activation and migration of local antigen-presenting cells (APCs), including Langerhans cells (LCs), upregulation in pleiotropic proinflammatory cytokines such as interleukin-1 (IL-1) and tumor necrosis factor-alfa (TNF-alpha) that can mediate a wide array of immune functions in addition to up-regulating protease expression. and chemokines that play a critical role on the one hand in attracting nonantigen-specific inflammatory cells such as neutrophils and on the other in attracting CD4+ T helper type 1 (Th1) cells that mediate most of the destruction in the cornea. CONCLUSIONS: In the last 25 years, we have seen our field develop from a descriptive stage into a new phase where the fundamental processes that mediate and effect corneal immunity are being accurately deciphered. It is anticipated that this new knowledge will allow development of specific molecular and genetic therapeutic strategies that could target critical steps in the immunopathogenesis of disease without the untoward side-effects of nonspecific generalized immune suppression that still remains the standard of care today.

Pseudomonas aeruginosa orbital phlegmon in a patient treated for myelodysplastic syndrome with concomitant Sjögren's syndrome.

Pseudomonas aeruginosa orbital infections have been described very rarely in patients with neutropenia after chemotherapy. We report the case of a woman with the unusual association of Sjögren's disease and myelodysplasia, who suffered from a Pseudomonas aeruginosa orbital phlegmon after chemotherapy for her myelodysplastic syndrome. Partial intestinal antibiotic decontamination with ciprofloxacine did not prevent the infection. She was treated successfully with intravenous ceftazidime, netilmicin and granulocyte-colony stimulating factor (G-CSF). The normalization of the granulocyte count seems to play a crucial role for recovery. We present the clinical and radiological findings, discuss the therapy and review the literature concerning ocular infections due to Pseudomonas. Other infections due to this germ in immunocompromised hosts are briefly reviewed.