New evidence for involvement of retroviruses in Sjögren's syndrome and other autoimmune diseases

Diversity of HLA-A2-Restricted and Immunodominant Epitope Repertoire of Human T-Lymphotropic Virus Type 1 (HTLV-1) Tax Protein: Novel Insights among N-Terminal, Central and C-Terminal Regions

The present study sought to search for the immunodominance related to the N-terminal, Central and C-terminal regions of HTLV-1 Tax using novel, cutting-edge peptide microarray analysis. In addition, in silico predictions were performed to verify the presence of nine amino acid peptides present along Tax restricted to the human leukocyte antigen (HLA)-A2.02*01 haplotype, as well as to verify the ability to induce pro-inflammatory and regulatory cytokines, such as IFN-γ and IL-4, respectively. Our results indicated abundant dose-dependent reactivity for HLA-A*02:01 in all regions (N-terminal, Central and C-terminal), but with specific hotspots. Furthermore, the results of fold-change over the Tax11–19 reactivity obtained at lower concentrations of HLA-A*02:01 reveal that peptides from the three regions contain sequences that react 100 times more than Tax11–19. On the other hand, Tax11–19 has similar or superior HLA-A*02:01 reactivity at higher concentrations of this haplotype. The in silico analysis showed a higher frequency of IFN-γ-inducing peptides in the N-terminal portion, while the C-terminal portion showed a higher frequency of IL-4 inducers. Taken together, these results shed light on the search for new Tax immunodominant epitopes, in addition to the canonic Tax11–19, for the rational design of immunomodulatory strategies for HTLV-1 chronic diseases.

Successful treatment of refractory neuroSjogren with Rituximab

We report a 47-year-old female with Sjogren's syndrome (SS) and severe weakness in her lower extremities refractory to cyclophosphamide therapy, who was treated with the monoclonal anti CD-20 antibody rituximab at a weekly dose of 375 mg/m2 for four consecutive weeks. Patient responded within few days of the first dose and her motor power in the lower extremities started to improve gradually along with progressive resolution of the paresthesias and dysesthesias. The improvement was sustained and progressive and eight months after the last dose, she was able to walk for 60 meters without aid or rest. Rituximab may be considered as an effective and promising novel therapy in SS patients with neurological involvement. Lupus (2007) 16, 521—523.

Virus-Induced Demyelination: The Case for Virus(es) in Multiple Sclerosis

Multiple Sclerosis (MS) is the most common demyelinating disease of man with over 400,000 cases in the United States and over 2.5 million cases worldwide. There are over 64,000 citations in Pubmed dating back as far as 1887. Much has been learned over the past 129 years with a recent burst in therapeutic options (mostly anti-inflammatory) with newer medications in development that are neuroprotective and/or neuroreparative. However, with all these advancements the cause of MS remains elusive. There is a clear interplay of genetic, immunologic, and environmental factors that influences both the development and progression of this disorder. This chapter will give a brief overview of the history and pathogenesis of MS with attention to how host immune responses in genetically susceptible individuals contribute to the MS disease process. In addition, we will explore the role of infectious agents in MS as potential “triggers” of disease. Models of virus-induced demyelination will be discussed, with an emphasis on the recent interest in human herpesviruses and the role they may play in MS disease pathogenesis. Although we remain circumspect as to the role of any microbial pathogen in MS, we suggest that only through well-controlled serological, cellular immune, molecular, and animal studies we will be able to identify candidate agents. Ultimately, clinical interventional trials that either target a specific pathogen or class of pathogens will be required to make definitive links between the suspected agent and MS.

Differential expression of a human endogenous retrovirus E transmembrane envelope glycoprotein in normal, psoriatic and atopic dermatitis human skin

BACKGROUND

Psoriasis is a common inflammatory skin disease characterized by uncontrolled proliferation of keratinocytes and recruitment of T lymphocytes into the skin. The possible role of human endogenous retroviruses (HERVs) in the induction of psoriasis has been suggested, based upon the previous observations of retrovirus-like particles in psoriasis from skin lesional plaques, urine and stimulated lymphocytes.

OBJECTIVES

To investigate the expression of HERV-E transmembrane envelope glycoprotein (HERV-E env) in normal, psoriatic and atopic human skin, and to examine the influence of ultraviolet (UV) B irradiation on HERV-E env expression in normal human epidermal keratinocytes.

METHODS

The analysis was performed on both skin biopsies and organotypic skin cultures using immunofluorescence and Western immunoblotting. UVB irradiation (312 nm) of cultured normal human keratinocytes was performed using a dose of 30 mJ cm(-2).

RESULTS

Positive staining was observed in most of the psoriatic and atopic skin samples, whereas only 15% of the normal skin samples were faintly positive. In addition, the pattern of expression of HERV-E env differed markedly in psoriasis vs. atopy. By Western blotting analysis, two main proteins of 54 and 57 kDa were detected in extracts of normal skin, normal keratinocyte cultures and reconstructed epidermis from psoriatic and normal punch biopsies. An increased level of expression of these proteins was noted in extracts from psoriatic vs. normal reconstructed epidermis. The overexpression of the 57-kDa protein in normal human cultured keratinocytes was dramatically reduced by UVB irradiation.

CONCLUSIONS

These data suggest for the first time that HERV-E env is expressed in normal and pathological human skin. Further studies are now required to elucidate the role of such viral proteins in the pathogenesis of psoriasis.

Autoantibodies to human endogenous retrovirus-K are frequently detected in health and disease and react with multiple epitopes

A number of studies have found increased levels of antibodies to human endogenous retroviruses (HERVs) in autoimmune rheumatic diseases. It is not clear whether this immune response is driven by the HERV itself or by cross-reactions with an exogenous virus or an autoantigen. To address this question, we examined the antibody response to the Env protein of two closely related members of the HERV-K family, HERV-K10 and IDDMK1,222. By immunoblotting of recombinant proteins, antibodies were found in 32-47% of 84 sera from patients with autoimmune rheumatic disease, and 29% of 35 normal controls. Epitope mapping with overlapping 15mers identified multiple reactive peptides on both antigens, with one (GKTCPKEIPKGSKNT) containing immunodominant epitope(s). By ELISA, the median titre of antibody to this peptide was significantly increased in 39 patients with SLE compared to 39 healthy controls and 86 patients with other rheumatic diseases (P < 0.003). We have shown that there is a high frequency of IgG antibodies to HERV-K env sequences in human sera, both in health and autoimmune rheumatic disease, and that the response is to multiple epitopes. This supports the hypothesis that the autoimmune response to HERV-K is antigen-driven and may be an early stage in the chain of events that leads to tolerance breakdown to other autoantigens.

Role of viruses in etiology and pathogenesis of multiple sclerosis

Multiple sclerosis (MS) is the most prevalent demyelinating disease of young adults, affecting an estimated 300,000 individuals in the United States alone. The majority of affected individuals have a relapsing–remitting course while a smaller subset has a more chronic–progressive presentation. Women are affected more often than men, a phenomenon associated with a number of auto-immune diseases. Although the etiology of MS is unknown, it is generally believed that genetic, immunologic, and environmental factors are involved. This chapter discusses these issues as they suggest that exogenous factors are associated with the pathogenesis of this disorder. Recently, the human herpes virus 6 (HHV-6) has received considerable attention as an infectious agent candidate that might be associated with the pathogenesis of MS. The chapter focuses on this agent and the data that support the role of this virus in MS disease pathogenesis. A model is proposed, whereby in genetically susceptible individuals, multiple viruses may trigger either a virus-specific or a cross-reactive auto-immune response that results in clinical MS. Epidemiologic evidence suggests that it is a multifactorial disease that develops as a result of host genetics, immune response, and environment.

Relationship between Sjögren's syndrome and human T-lymphotropic virus type I infection: follow-up study of 83 patients

We have previously demonstrated a high prevalence of Sjögren's syndrome (SS) in patients with human T-lymphotropic virus type I-associated myelopathy (HAM) in Nagasaki prefecture. The present follow-up study compared the clinical and laboratory findings of SS with or without human T-lymphotropic virus type I (HTLV-I) antibody in this endemic area for HTLV-I infection. We investigated the clinical and laboratory manifestations in 83 patients with SS and HAM, including histologic examination of labial salivary glands and the prevalence of SS in patients with HAM. Definite SS was diagnosed in 13 out of 20 patients with HAM when the European Community criteria were used. The density of mononuclear cell infiltration in labial salivary glands was higher in HTLV-I-seropositive patients with SS (including patients with HAM) than in HTLV-I-seronegative patients. The volume of saliva and lacrima determined by the Schirmer or Saxon test was lower than normal but was not different among SS-HTLV-I-seronegative patients, HTLV-I-seropositive patients without HAM, and HTLV-I-seropositive patients with HAM. The proportions of patients positive for antinuclear antibody (ANA) and anti-SS-A (Ro) antibody or anti-SS-B (La) antibody were similar in the three groups. However, the low volume of saliva and the frequency of ANA in SS correlated with the degree of mononuclear cell infiltration in labial salivary glands. Our results suggested that HTLV-I infection is related to SS and that laboratory and clinical findings in SS closely correlate with the degree of mononuclear cell infiltration in the salivary glands.

Immune responses to antigens of human endogenous retroviruses in patients with acute or stable multiple sclerosis

A possible role for human endogenous retroviruses (HERV) in the pathogenesis of MS was investigated by analyzing HERV peptides-stimulated proliferation and cytokine production in MS patients with acute (AMS) or stable (SMS) disease. HERV peptides specific-proliferation and type 1 cytokine production by peripheral blood mononuclear cells was observed in AMS but not in SMS individuals, in whom a type 2 cytokine profile dominates. HERV peptides-stimulated immune responses were modified by changes in disease expression; mediated by CD4+ T lymphocytes; and not related to HLA class II molecules. These data suggest the possibility of a pathogenic role for HERV and HERV-specific immune responses in MS.

Influence of amino acid substitutions on antigenicity of immunodominant regions of the HTLV type I envelope surface gylcoprotein: a study using monoclonal antibodies raised against relevant peptides

By the use of sera of human T cell leukemia virus type I (HTVL-I)-infected individuals it was shown that amino acid substitutions at positions 192 (proline to serine) and 250 (serine to proline) in major immunodominant regions (175-199 and 239-261) of the surface envelope glycoprotein (gp46) of the virus may influence the humoral response. Since human sera are polyclonal in nature, one cannot readily discriminate between an immunoglobulin-specific recognition and multiple bindings of diverse antibodies. To overcome this difficulty we generated murine monoclonal antibodies to synthetic peptides mimicking all or portions of these gp46 regions. The reactivity of some of these antibodies to synthetic peptides harboring (or not harboring) the preceding amino acid substitutions at position 192 or 250, to denatured gp46 by Western blotting, and to live (variously substituted) HTLV-I-infected cells, combined with blocking experiments with various peptides, allow us to conclude that the major epitopes (positions 183-191, 190-197, 190-199, and 246-252) in the two immunodominant regions may elicit different antibody responses according to their sequences. It is worth noting that in a reporter gene inhibition assay, it was found that a neutralizing monoclonal antibody (MF1), the epitope for which is located between residues 190 and 197, had a high level of activity when cells (2060) harboring a gp46 with proline at position 192 were used and had no activity toward cells (1010) with a serine at this position. Therefore our results establish that certain amino acid substitutions of gp46 may drastically affect the antigenicity of the molecule and the biological activity of the antibodies elicited.

Reactivity of sera from systemic lupus erythematosus and Sjögren's syndrome patients with peptides derived from human immunodeficiency virus p24 capsid antigen

We have previously demonstrated that about one-third of patients with either Sjögren's syndrome (SS) or systemic lupus erythematosus (SLE) react to human immunodeficiency virus (HIV) p24 core protein antigen without any evidence of exposure to, or infection with, HIV itself. Herein, we further characterize the specificity of this reaction using enzyme-linked immunosorbent assay to peptides representing fragments of p24. Characteristic epitope-specific profiles were seen for SS and SLE patients. SS patients had significantly increased responses to peptides F (p24 amino acids 69 to 86) and H (amino acids 101 to 111) and diminished reactivity to peptides A (amino acids 1 to 16) and P (amino acids 214 to 228). SLE patients had increased reactivity to peptides E (amino acids 61 to 76), H, and P. Utilization of peptide P hyporeactivity as the criterion to select for SS patients results in a screen that is moderately sensitive (64%) and specific (79.3%). Adding hyperreactivity to one other peptide (F or H) as an additional criterion yields an expected decrease in sensitivity (to 41%) while increasing specificity (to 93.1%). All sera-reactive peptides from regions of known structure of HIV p24 were located in the apex of the p24 molecule. Thus, the specificity of the peptide reactivities described here indicates a specific pattern of a nonrandom cross-reactivity between HIV type 1 p24 and autoimmune sera which may be partially syndrome specific. The future focus of our work will be to optimize assays of the peptide as diagnostic tools.

Sjögren's syndrome in women presenting with chronic dyspareunia

OBJECTIVE

To identify cases of Sjögren's syndrome among women with chronic dyspareunia who did not already have a diagnosed rheumatological disorder.

DESIGN

Prospective recruitment over 12 months.

SETTING

Tertiary referral service for the assessment of vulval disease.

PARTICIPANTS

Women with chronic dyspareunia who had musculoskeletal symptoms, Raynaud's phenomenon or symptoms of ocular or oral dryness.

METHODS

The women underwent a Schirmer tear test and a comprehensive auto-antibody screen including latex fixation test for rheumatoid factor, antinuclear, anti-Ro, anti-La and anti-salivary duct antibodies. A labial salivary gland biopsy and vaginal biopsy were taken for routine histological analysis.

MAIN OUTCOME MEASURES

Cases of definite and probable Sjögren's syndrome were identified using the European criteria.

RESULTS

Eleven women were assessed for features of Sjögren's syndrome. Four had definite primary Sjögren's syndrome, two had probable primary Sjögren's syndrome and one had probable secondary Sjögren's syndrome. Among these seven women the median duration of vaginal symptoms was seven years (range 0.25-20), of ocular symptoms was one year (range 0.25-2) and of oral symptoms was 1.5 years (range 0-6). In all but one woman dyspareunia presented before ocular or oral symptoms, often by many years.

CONCLUSIONS

Although well-recognised as a feature of established Sjögren's syndrome, this study emphasises that chronic dyspareunia can be a presenting feature in these women, antedating the emergence of ocular or oral symptoms by many years. Symptoms of ocular or oral dryness, Raynaud's phenomenon or musculoskeletal symptoms should be sought in women with chronic dyspareunia to identify those who merit further investigation.

Arthritis syndromes associated with human T cell lymphotropic virus type I infection

Arthritis syndromes occur associated with HTLV-I infection both in the presence and in the absence of clinical ATL, and polyarthritis may be the presenting manifestation of HTLV-I-associated ATL. In both clinical settings, HTLV-I-infected T cells home to affected joints, and tax-transgenic mouse studies have suggested a pathogenic role for the HTLV-I tax gene in inducing synovial cell proliferation in HAA. Understanding the pathogenesis of rheumatoid arthritis-like arthritis syndromes that occur in the setting of HTLV-I infection should also provide insights into understanding of cellular and molecular mechanisms of synovial cell proliferation in HTLV-I-negative rheumatoid arthritis.

Among all human T-cell leukemia virus type 1 proteins, tax, polymerase, and envelope proteins are predicted as preferential targets for the HLA-A2-restricted cytotoxic T-cell response

The human T-cell leukemia virus type 1 (HTLV-1) is a human retrovirus associated with two diseases for which no successful treatment is yet available; the development of a vaccine is therefore an important issue. Since HTLV-1 is a persistent virus, an efficient vaccine will probably require a cytotoxic T-lymphocyte (CTL) response in addition to the production of antibodies. To identify potential CTL epitopes, we have selected, within all of the HTLV-1 proteins, nonapeptides containing anchor residues required for association with HLA-A2 molecules (residues at positions 2 and 9), which is the most frequently occurring A allele in all human populations. A set of 111 peptides was synthetized and tested in vitro in two assembly assays using processing-defective T2 cells. Anchor motifs selected were those containing two major anchor residues (L2/M2/12-V9/L9/I9) (one letter amino-acid code) and those including tolerated anchor residues (V2/A2/T2 and/or A9/M9/T9). The analysis of the binding capacity of the peptides confirms the high efficiency of the L2-V9 anchor motif and shows that a systematic research of potential binding peptides should exclude peptides containing known detrimental residues rather than select only peptides with known favored residues. We show that 39 peptides representative of all the HTLV-1 proteins are able to bind to HLA-A2 molecules. Strong binder peptides which are very likely good CTL epitopes were identified in three HTLV-1 proteins, Tax, envelope, and polymerase. Three of the strong binder peptides correspond to previously described HLA-A2-restricted CTL epitopes in the Tax protein, and two others are localized in a domain of the viral envelope recognized by natural neutralizing antibodies. This latter result has important implications for the development of an anti-HTLV-1 vaccine.

Antibodies against retroviral proteins and nuclear antigens in a subset of idiopathic CD4+ T lymphocytopenia patients

Idiopathic CD4+ T lymphocytopenia (ICL) is an immunodeficiency syndrome characterized by severe depletion of CD4+ T lymphocytes, but in which human immunodeficiency virus cannot be detected. Peripheral blood mononuclear cells (BPMCs) from an ICL patient were cocultured with HUT78 T-lymphoblastoid cells, and an acute cytopathic effect and formation of multinucleated cells were observed. A human intracisternal A-type retroviral particle designated HIAP-II was detected in cells surviving the acute cytopathic effect. Eight of 13 ICL patients in a blinded screen of a serological panel provided by the National Centers for Disease Control and Prevention (CDC) had serum antibodies that specifically reacted with HIAP-II associated proteins by Western immunoblotting. None of 19 control sera in the panel that were unreactive with HIV Gag proteins produced a positive result on HIAP-II immunoblots. Comparable results were obtained in a blinded screen of a second CDC serological panel. Sera from 8 of 14 ICL patients in the second serological panel were positive for antinuclear autoantibodies (ANAs) commonly observed in patients with systemic autoimmune diseases. These results suggest the possible involvement of an A-type retrovirus or autoimmunity in development of ICL in a subset of patients.

Acquired immune deficiency syndrome caused by hyperimmunity

Normal immunocytes including T and B cells are equilibrated by a reciprocal attacking mechanism called a network. Continuous disequilibrium of this network results in general immunodeficiency with oligo- and polyclonal hyperimmunity, for example, T-cell activation due to spontaneous reticuloendotheliosis, paraneoplastic autoimmune syndromes, and human immunodeficiency virus infection. In these disorders, reciprocal self-reactivity, including autologous graft-versus-host reaction, plays a role in the immunodeficiency. A priori self-targeting immunity is a key mechanism to explain autoimmunity in the acquired immunodeficiency syndrome. In the treatment of the immunodeficiency due to hyperimmunity, I propose immunological suppression by agents and reconstitution of the network by bone-marrow transplantation.

Current concepts of autoimmune exocrinopathy: immunologic mechanisms in the salivary pathology of Sjögren's syndrome

Sjögren's syndrome is a systemic autoimmune disorder characterized by symptoms of oral and ocular dryness and a chronic, progressive loss of salivary and lacrimal function. The exocrine involvement is the result of a focal, peri-ductal mononuclear cell infiltrate and the subsequent loss of secretory epithelial cells. The mechanisms of this autoimmune exocrinopathy are not understood fully. Many recent investigations have described alterations in a number of immune mediators within the salivary glands. These studies provide new insights into the immune regulation of normal salivary gland functions and the mechanisms of gland damage in Sjögren's syndrome.

Antibody reactivity to the HRES-1 endogenous retroviral element identifies a subset of patients with systemic lupus erythematosus and overlap syndromes. Correlation with antinuclear antibodies and HLA class II alleles

OBJECTIVE

To evaluate the correlation between the presence of antibodies to an endogenous retroviral element-encoded nuclear protein autoantigen, HRES-1, and the presence of other antinuclear antibodies and HLA class II alleles in patients with systemic lupus erythematosus (SLE) and overlap syndromes.

METHODS

Antibody reactivities to native and recombinant proteins and synthetic peptides were assessed by counterimmunoelectrophoresis, enzyme-linked immunosorbent assay, and Western blotting. HLA class II alleles were determined by oligonucleotide typing.

RESULTS

Forty-eight percent of the 153 patients with autoimmune disease, and 52% of the subgroup with SLE, had HRES-1 antibodies. In contrast, 3.6% of 111 normal donors, and none of 42 patients with the acquired immunodeficiency syndrome or 50 asymptomatic human immunodeficiency virus 1-infected patients, had HRES-1 antibodies. Chi-square analyses revealed a significant association between anti-HRES-1 and anti-RNP and an inverse correlation between HRES-1 and Ro/La autoantibodies in patients with SLE or overlap syndromes. Antigenic epitopes of HRES-1 and the retroviral gag-related region of the 70-kd protein component of U1 small nuclear RNP, which share 3 consecutive highly charged amino acids (Arg-Arg-Glu), an additional Arg, and functionally similar Arg/Lys residues, represent cross-reactive epitopes between the two proteins. Selective removal of HRES-1 antibodies from sera of HRES-1-seropositive/RNP-seropositive patients by absorption on recombinant HRES-1/glutathione-S-transferase-conjugated agarose beads had no effect on anti-RNP reactivities. A comparative multivariate analysis of HLA class II genes revealed a differential segregation of DQB1 alleles in HRES-1-seropositive versus HRES-1-seronegative patients (P = 0.04). While a relative increase of DQB1*0402 among HRES-1-seropositive patients was noted across ethnic groups (P = 0.02), a decrease of DQB1*0201 and DQB1*0301 was found in white HRES-1-seropositive patients (P = 0.04).

CONCLUSION

Autoantibodies to HRES-1 are detectable in a distinct subset of patients with autoimmune disease, primarily in those who do not have antibodies to Ro and La. Anti-HRES-1 and anti-RNP reactivities are mediated by cross-reactive but separate antibody molecules. HLA-DQB genes, rather than HLA-DRB or DQA genes, may have a more significant influence on generation of these antinuclear autoantibodies.

Significance of extractable nuclear antigens in childhood autoimmune liver disease

Antinuclear antibody (ANA) is found in connective tissue disorders and in autoimmune liver disease. While ANA-positive connective tissue disorders are subdivided according to possession of specific antibodies to extractable nuclear antigens (ENA) (anti-ribonucleoprotein (anti-RNP), anti-Smith (anti-Sm), anti-Ro, anti-La), little is known about the presence and significance of ENA in autoimmune liver disease. To investigate this, we have tested 35 children with autoimmune hepatitis (AIH) (19 ANA and/or smooth muscle antibody-positive (ANA/SMA+ve); 16 liver kidney microsomal 1-positive (LKM-1 + ve)) and 14 with ANA/SMA+ve autoimmune sclerosing cholangitis (ASC), using both double dimension immunodiffusion and ELISAs. Eighty children with non-autoimmune liver disease (20 alpha 1-antitrypsin deficiency, 20 Wilson's disease, 20 Alagille's syndrome and 20 chronic hepatitis B virus infection) and 20 healthy controls were also tested. ENA were detected in seven (20%) patients with AIH: two ANA-positive, one SMA-positive and four LKM-1-positive. Three were positive for anti-Sm, two for anti-La, one for anti-Sm/anti-La and one for anti-Sm/anti-La/anti-Ro. ENA-positive had more severe liver disease than ENA-negative patients (P < 0.03). ENA were not detected in ASC, non-autoimmune liver diseases and controls. Our results indicate that ENA reactivity, including anti-Sm and anti-La, characteristic of systemic lupus erythematosus and Sjögren's syndrome, respectively, are present in some patients with AIH even in the absence of ANA, and may characterize a particularly severe form of the disease.

IgA antiphospholipid antibodies in HTLV-1-associated tropical spastic paraparesis

A retrovirus, human T cell lymphotropic virus type 1 (HTLV-1), is an essential but not a sufficient aetiologic factor for tropical spastic paraparesis (TSP). Because some TSP patients have biological false positive tests for treponemal infections (BFP-STS), we used ELISA to study BFP-STS and anticardiolipin antibodies in 42 Jamaican TSP patients. The data indicate that in TSP anticardiolipin antibodies occur in about 26% of patients, are associated with biological false positive treponemal serology, are relatively restricted to the IgA isotype and may be induced by HTLV-1 or other nontreponemal infections.

Viruses, virulence and pathogenicity

Pathogenicity is a complex process with stringent requirements of both the host cell and the infecting virion. Among these requirements are a port of entry into host cells, a means of replication for the virus, and a means by which infection damages host cells. Damage to the host can result from multiple mechanisms including transformation, suppression of cellular metabolism, apoptosis, autoimmune responses directed against infected or uninfected tissues, or by molecular mimicry. In the attempt to identify new associations between viral infection and disease, investigators should be mindful that variable host factors as well as viral infection may be required for pathogenesis. Efforts to associate specific viral infections with specific diseases may be obscured by final common pathways through which multiple agents damage host cells in similar ways.