Detection of cell-free Epstein-Barr virus DNA in serum during acute infectious mononucleosis

Detection of avian oncogenic Marek's disease herpesvirus DNA in human sera

The avian herpesvirus Marek's disease virus (MDV) has a worldwide distribution and is responsible for T-lymphoma in chickens. The question as to whether MDV poses a public health hazard to humans was first raised when the virus was isolated in 1967. However, no irrefutable results have been obtained in immunological and virological studies. We used a nested-PCR to detect MDV DNA in human serum samples. A total of 202 serum samples from individuals exposed and not exposed to poultry was tested by nested-PCR for a target sequence located in the MDV gD gene. The assay system was specific and sensitive, making it possible to detect a single copy of the target sequence. Forty-one (20%) of the 202 serum samples tested positive for MDV DNA. The prevalence of MDV DNA was not significantly different in the group exposed to poultry and the group not exposed to poultry. There was also no difference due to age or sex. Alignment of the 41 gD sequences amplified from human sera with eight gD sequences amplified from MDV-infected chicken sera showed a maximum nucleotide divergence of 1.65%. However, four 'hot-spot' mutation sites were identified, defining four groups. Interestingly, two groups contained only human MDV-gD sequences. The status of the MDV genome detected in human blood is discussed.

Usefulness of quantitative real-time polymerase chain reaction in following up patients with Epstein-Barr virus infection after liver transplantation

BACKGROUND

Post-transplant lymphoproliferative disease (PTLD), which is mainly induced by Epstein-Barr virus (EBV) infection, is a cause of significant morbidity and mortality for patients undergoing liver transplantation, especially when it is detected at such an advanced stage as monoclonal malignant lymphoma.

METHODS

In this series, 6 of 22 liver transplant patients suffered from EBV infection. We tested quantitative DNA (Qt-DNA) by real-time polymerase chain reaction (PCR), qualitative DNA in plasma (Q1-pDNA) by PCR, and EBV-encoded mRNA 1 (EBER 1) by in situ hybridization to clarify which of them is a better marker for the early diagnosis and prediction of EBV-associated disorders.

RESULTS

Four had signs or symptoms of PTLD, but 2 did not develop individualized lymphoid lesions. In all patients, both Qt-DNA and EBER 1 exceeded the cut-off level of 10(2.5) copies/microg DNA and 0.002%, respectively, at the time of diagnosis. In 2 patients, when Qt-DNA had a poor decline, EBER 1, even if it seemed to decrease after antiviral therapy, increased again after a few months and the clinical symptoms recurred. In 2 patients, Qt-DNA and EBER 1 increased again after a few months of antiviral therapy, and Q1-pDNA remained positive, whereas, in 3 patients, no reaction of EBV could be detected once Q1-pDNA became negative, even after the cessation of therapy.

CONCLUSIONS

These results suggest that real-time PCR for Qt-DNA was more sensitive to the real-time activity of EBV and that Q1-pDNA could indicate when to stop antiviral therapy.

Development of a real-time quantitative assay for detection of Epstein-Barr virus

With the use of real-time PCR, we developed and evaluated a rapid, sensitive, specific, and reproducible method for the detection of Epstein-Barr virus (EBV) DNA in plasma samples. This method allowed us to screen plasma and serum samples over a range between 100 and 10(7) copies of DNA per ml using two sample preparation methods based on absorption. A precision study yielded an average coefficient of variation for both methods of less than 12%, with a coefficient of regression for the standard curve of a minimum of 0. 98. We detected EBV DNA in 19.2% of plasma samples from immunosuppressed solid-organ transplant patients without symptoms of EBV infections with a mean load of 440 copies per ml. EBV DNA could be detected in all transplant patients diagnosed with posttransplant lymphoproliferative disorder, with a mean load of 544,570 copies per ml. No EBV DNA could be detected in healthy individuals in nonimmunosuppressed control groups and a mean of 6,400 copies per ml could be detected in patients with infectious mononucleosis. Further studies revealed that the inhibitory effect of heparinized plasma could be efficiently removed by use of an extraction method with Celite as the absorbent.

Oligo-monoclonal immunoglobulins frequently develop during concurrent cytomegalovirus (CMV) and Epstein-Barr virus (EBV) infections in patients after renal transplantation

In the present study we report that the appearance of oligo-monoclonal immunoglobulins (oligoM-Igs) in the sera of transplanted individuals is concurrent with the detection of coincident active CMV infection and EBV replication. Eighty-four renal allograft patients were monitored with respect to CMV isolation, to CMV conventional serology and humoral response against the EBV trans-activator ZEBRA (an immediate-early antigen also called BZLF1). Titration of anti-ZEBRA antibodies (IgG and IgM) and amount of EBV DNA in serum were evaluated. Using the combination of four techniques (agarose gel electrophoresis, analytical isoelectric focusing, high resolution immunoelectrophoresis, immunofixation electrophoresis), oligoM-Igs were found in 25% of patients after allografting and significantly associated with rejection episodes (P < 0.001). Twenty out of 23 (86%) concurrent CMV/EBV infections were associated with serum oligoM-Igs (P < 0.001). One can thus reasonably assume that a sustained EBV replication following iatrogenic immunosuppression can promote the immunoglobulin heavy chain expression in EBV-infected B lymphocytes. The proliferation of immunoglobulin-secreting clones might occur after active CMV infection, through a transient over-immunosuppression or via immune subversion.

Detection of Epstein-Barr virus (EBV) genomes in the serum of patients with EBV-associated Hodgkin's disease

DNA from malignant cells is present in the serum/plasma of cancer patients and DNA from this source is amenable to analysis by polymerase chain reaction (PCR). In the present study, we evaluated whether Epstein-Barr virus (EBV) DNA is present in the serum of patients with EBV-associated Hodgkin's disease (HD). Using conventional PCR, EBV DNA was detected in serum from 30/33 patients with EBV-associated HD but in only 6/26 patients with non-EBV-associated disease (p < 0.001). Samples from healthy individuals were negative and only 5/12 infectious mononucleosis samples were positive. Real-time quantitative PCR was subsequently employed to determine the concentration of EBV DNA present in serum; among positive samples the level ranged from 1 to 705 copies per 125 microliter of serum. Post-treatment samples from 5/14 cases with EBV-associated HD contained detectable EBV DNA; analysis of this small group of cases suggests that positivity in post-treatment samples correlates with risk factors indicative of a poor prognosis. Overall, our results are consistent with the notion that DNA from Reed-Sternberg cells is present in the serum of HD patients, and further suggest that serum EBV should be evaluated as a prognostic marker. Int. J. Cancer (Pred. Oncol.) 84:442-448, 1999.

Detection and typing of lymphotropic herpesviruses by multiplex polymerase chain reaction

A multiplex nested-polymerase chain reaction (PCR) method was developed for the simultaneous detection and typing of all human lymphotropic herpesviruses described to date, including Ebstein Barr virus (EBV), cytomegalovirus (CMV), human herpesvirus 6, variants A and B (HHV6-A, HHV6-B), human herpesvirus 7 (HHV7) and human herpesvirus 8 (HHV8). Oligonucleotide primers were designed to amplify a highly conserved region within the DNA polymerase gene. Each reaction component and thermal cycling parameters were thoroughly standardized to achieve optimal specificity and sensitivity for the PCR assay, which was estimated at about 10-100 molecules for each virus. An internal control, consisting of 100 molecules of a cloned fragment of the porcine pseudorabies herpesvirus (PrV) genome, was included to detect false negative results. To assess suitability and clinical application of the multiplex PCR method, a total of 35 well-characterized specimens, including Kaposi's sarcoma skin lesions, serum, cerebrospinal fluid, saliva and urine samples, were tested. Results obtained suggest this technique could be applied as a sole diagnostic tool in several clinical settings in which herpesviral infection is suspected and differential diagnosis required, avoiding the need to test specimens by separate PCR methods.

High Epstein-Barr virus serum load and elevated titers of anti-ZEBRA antibodies in patients with EBV-harboring tumor cells of Hodgkin's disease

Hodgkin's disease is commonly associated with EBV latent infection. The incidence of EBV reactivation (active infection or EBV infection with replicative cycle) was evaluated in a series of 30 patients with untreated Hodgkin's disease (except for one case with chronic lymphocytic leukemia) by quantitation of EBV DNA and titration of anti-ZEBRA antibodies in serum samples. DNA was detected in serum (>2.5 x 10(2) genomes/ml) in 15 of 30 patients and was more frequent in Hodgkin's disease with EBV-positive Reed-Sternberg cells (10/12) than in EBV-negative cases (5/18), (P< 0.01). Of interest was the demonstration that viremia correlated well with increased titers of anti-ZEBRA IgG and/or standard serological profiles of EBV reactivation (12/15), (P < 0.05). However the lack of EBV replicative cycle in Reed-Sternberg cells (negative for ZEBRA antigen and early antigen BHLF1) suggests that the viral replication occurs in a nonneoplastic cell compartment rather than in tumor cells. The measurement of EBV DNA loads and the titration of anti-ZEBRA antibodies shed new lights on the link between activation of EBV replication and Hodgkin's disease: these serological markers together with the determination of the EBV status of the tumor suggest that replication of the viral genome occurs with a decreased efficiency of the immune system, thus allowing progression of the tumor.

Quantitative analysis of Epstein-Barr virus load by using a real-time PCR assay

To measure the virus load in patients with symptomatic Epstein-Barr virus (EBV) infections, we used a real-time PCR assay to quantify the amount of EBV DNA in blood. The real-time PCR assay could detect from 2 to over 10(7) copies of EBV DNA with a wide linear range. We estimated the virus load in peripheral blood mononuclear cells (PBMNC) from patients with symptomatic EBV infections. The mean EBV-DNA copy number in the PBMNC was 10(3.7) copies/microg of DNA in patients with EBV-related lymphoproliferative disorders, 10(4.1) copies/microg of DNA in patients with chronic active EBV infections, and 10(2.2) copies/microg of DNA in patients with infectious mononucleosis. These numbers were significantly larger than those in either posttransplant patients or immunocompetent control patients without EBV-related diseases. In a patient with infectious mononucleosis, the virus load decreased as the symptoms resolved. The copy number of EBV DNA in PBMNC from symptomatic EBV infections was correlated with the EBV-positive cell number determined by the in situ hybridization assay (r = 0.842; P < 0.0001). These results indicate that the real-time PCR assay is useful for diagnosing symptomatic EBV infection and for monitoring the virus load.

Common and emerging infectious causes of hematological malignancies in the young

Comparative epidemiological studies have for a long time suggested a link (or links) between infectious agents and hematological malignancies in the young. Identification of Epstein-Barr virus (EBV) as the major cause of specific subtypes of Burkitt's lymphoma and Hodgkin's disease 20 and 10 years ago, respectively, and the recent involvement of human T-cell leukemia virus in non-Hodgkin's lymphomas of the T-cell lineage in young adults in Jamaica have given further credit to early presumptions that these diseases have an infectious etiology. The spectrum of possibly involved viruses: old, EBV, and new, herpesviruses 6, 7 and 8, and unknown retroviruses - as well as the list of partially or totally unresolved disease entities: Hodgkin's disease in adolescents, non-Hodgkin's lymphomas in the immunocompromised, and acute lymphocytic leukemia - is rapidly expanding. Both direct and indirect transforming effects of the above-mentioned viruses are being rapidly disclosed. However, the complex interaction between the different viruses and other causes of hematological malignancies in the young guarantees that many things remain to be discovered also in the future.

Epstein-Barr virus latent infection in vivo

Epstein-Barr virus (EBV) is an exclusively human herpes virus which is recognised as the causative agent of infectious mononucleosis and which is implicated in the aetiology of several cancers. However, it is particularly remarkable that this virus is harboured without causing symptoms for the lifetime of most immunocompetent adults. Virus and host have co-evolved over millions of years, achieving a balance between viral persistence and immune control. It is this dynamic equilibrium which is the focus of this review. The main site of viral persistence is within latently infected lymphocytes, although infectious virus is also released into saliva from productively infected cells in the oropharynx. In vitro, EBV efficiently transforms resting B cells to activated, perpetually dividing lymphoblasts. These express a repertoire of eight viral antigens, several of which have been found to be targets for cytotoxic T cell (CTL) responses in healthy carriers. Transformed lymphoblasts are susceptible to immune control in vivo, and are abundant only during primary infection or in individuals with impaired cell mediated immunity. Other types of viral latent infection have been identified in malignant cell lines, in which EBV expresses a more restricted range of antigens. These also may have their in vivo equivalents during natural infection in healthy carriers. It is likely that the virus evades elimination by the immune system by establishing infection in non-activated, relatively non immunogenic B cells, in which the main CTL target antigens are not expressed. Copyright 1997 John Wiley & Sons, Ltd.

Detection of Epstein-Barr viral DNA in serum using rapid-cycle PCR

Our study describes the comparison of a rapid nested PCR assay to standard serology techniques for the detection of Epstein-Barr virus (EBV) in serum. The sera of 81 patients with suspected EBV infection were analyzed; 54 were positive for one or more of the standard serology markers, i.e., IgM viral capsid antigen (VCA), IgG-VCA, Epstein-Barr nuclear antigen 1 (EBNA-1), and early antigen (EA), and 27 were negative for all serology markers. The sera from 15 normal healthy blood donors were also included. No EBV DNA was detected in any of the 15 blood donor samples or in any of the 27 samples with negative serology results. Eleven samples (20%) of the 54 with positive EBV serology results were positive for EBV DNA. Of these samples, 9 were EBV IgM-VCA positive and anti-EBNA negative, suggesting acute infection. One of the 11 samples had high titers of IgM-VCA, IgG-VCA, anti-EBNA, and anti-EA. The last of the 11 samples was from a patient with acute infectious mononucleosis without sufficient sample volume for EBV serology testing. Seventeen of the total 96 samples from the study were IgM-VCA positive and anti-EBNA negative and 9 of these 17 samples (53%) tested positive for EBV DNA. These data suggest that the detection of EBV DNA by PCR in serum may be a useful indicator of active infection rather than latent virus.

Lytic Replication of Epstein-Barr Virus in the Peripheral Blood: Analysis of Viral Gene Expression in B Lymphocytes During Infectious Mononucleosis and in the Normal Carrier State

Epstein-Barr virus (EBV) has been shown to establish latency in resting B lymphocytes of the peripheral blood. This creates a virus reservoir in contrast to lytic virus replication, which is thought to be restricted to differentiated epithelial cells in vivo. So far, the route of transmission between B cells and the production of progeny virus in the epithelial tissue has remained unclear. Reverse transcriptase–polymerase chain reaction (RT-PCR) and immunohistochemistry analysis of 16 patients with acute infectious mononucleosis (IM) and 25 healthy seropositive donors was performed to detect lytic replication gene products in B lymphocytes of the peripheral blood. Transcriptional activity was found in peripheral blood B lymphocytes (PBLs) for BZLF1 in 88%, BALF2 in 50%, and BcLF1 in 25% of the tested IM patients. All positive results were further confirmed in enriched B-cell populations by antigen determination using immunostaining with the APAAP technique. Furthermore, we detected transcripts for BZLF1 in 72% and for BALF2 in 16% of peripheral B lymphocytes of healthy seropositive donors. In contrast to patients with IM, no signals for BcLF1 were ever found in healthy seropositive donors. In these individuals, lytic replication of EBV is probably restricted by immunologic and gene regulatory mechanisms, whereas in the absence of immunologic control, reflected here by IM patients, the production of infectious virus becomes visible in PBLs.

Lytic Replication of Epstein-Barr Virus in the Peripheral Blood: Analysis of Viral Gene Expression in B Lymphocytes During Infectious Mononucleosis and in the Normal Carrier State

Epstein-Barr virus (EBV) has been shown to establish latency in resting B lymphocytes of the peripheral blood. This creates a virus reservoir in contrast to lytic virus replication, which is thought to be restricted to differentiated epithelial cells in vivo. So far, the route of transmission between B cells and the production of progeny virus in the epithelial tissue has remained unclear. Reverse transcriptase–polymerase chain reaction (RT-PCR) and immunohistochemistry analysis of 16 patients with acute infectious mononucleosis (IM) and 25 healthy seropositive donors was performed to detect lytic replication gene products in B lymphocytes of the peripheral blood. Transcriptional activity was found in peripheral blood B lymphocytes (PBLs) for BZLF1 in 88%, BALF2 in 50%, and BcLF1 in 25% of the tested IM patients. All positive results were further confirmed in enriched B-cell populations by antigen determination using immunostaining with the APAAP technique. Furthermore, we detected transcripts for BZLF1 in 72% and for BALF2 in 16% of peripheral B lymphocytes of healthy seropositive donors. In contrast to patients with IM, no signals for BcLF1 were ever found in healthy seropositive donors. In these individuals, lytic replication of EBV is probably restricted by immunologic and gene regulatory mechanisms, whereas in the absence of immunologic control, reflected here by IM patients, the production of infectious virus becomes visible in PBLs.

Epithelial cell polarization is a determinant in the infectious outcome of immunoglobulin A-mediated entry by Epstein-Barr virus

Diseases of the nasopharyngeal epithelium due to Epstein-Barr virus (EBV) infection typically occur in chronic virus carriers with preexisting virus-specific antibodies. In vitro studies have shown that EBV-specific immunoglobulin A (IgA) promotes infection of human epithelial cells, otherwise refractory to EBV, via the polymeric immunoglobulin receptor (pIgR). To determine if EBV similarly exploits IgA transport mechanisms in vivo, we examined the fate of IgA-EBV complexes in the blood of mice, where pIgR-mediated transcytosis of IgA immune complexes through hepatocytes eliminates exogenous antigens from the circulation. By PCR analysis we showed hepatobiliary transport of IgA-EBV in viremic mice, but without detectable hepatocellular infection by immunostaining. Because efficient transport of EBV immune complexes might avert an infectious outcome, we modulated the transcytotic pathway in polarized Madin-Darby canine kidney (MDCK) cells transfected with pIgR to determine the effect on viral antigen expression. Like hepatocytes in vivo, MDCK cells in polarized monolayers translocated IgA-EBV from the basal cell face into apical medium without evidence for infection. However, when exposed to IgA-EBV as unpolarized single-cell suspensions, MDCK cells expressed EBV immediate-early and early antigens. These results suggest that pIgR-mediated transcytosis of pIgA-EBV through epithelium facilitates endogenous spread of EBV in long-term virus carriers, with infection being confined to cells with altered polarity from prior cytopathology.

Epstein-Barr virus DNA in serum after liver transplantation--surveillance of viral activity during treatment with different immunosuppressive agents

In immunocompromised HIV-infected and transplanted patients, there is a risk of developing Epstein-Barr virus (EBV)-associated lymphoproliferative disorders (LPD) and lymphomas. EBV has previously been detected by the polymerase chain reaction (PCR) in cerebrospinal fluid from all AIDS patients with EBV-associated cerebral lymphomas. We therefore thought it would be of interest to determine whether transplant patients with extracerebral EBV-associated LPD have detectable EBV genomes in serum. Nested PCR (nPCR) showed that 58% (18/31) of liver transplant (LTX) patients had EBV DNA in 17% (21/125) of serum samples obtained within the first 3 months after LTX. In 39% (7/18) of the patients, the first EBV nPCR-positive sample was found within 2 weeks post-LTX. Basic immunosuppression with cyclosporin A or FK506 did not seem to influence the frequency of detectable EBV genomes in serum. In contrast, positive EBV nPCR correlated to secondary OKT3 treatment for severe acute rejection (P = 0.009). EBV-associated malignant lymphoma developed in three patients 2-6 months post-LTX. In all of them, EBV DNA was amplifiable within 12-14 days after LTX. The EBV antibody titers were not directly related to detectable EBV DNA in serum. We conclude that monitoring of LTX patients receiving increased immunosuppression by nPCR for EBV DNA in serum may help in the early identification of those at risk of developing EBV-associated LPD.

Detection of Epstein-Barr virus genome in ocular tissues

BACKGROUND

Epstein-Barr virus (EBV) is a ubiquitous mucosal pathogen with a propensity for lifelong, asymptomatic persistence. Because of reported association between EBV and ocular inflammatory disorders, we tested ocular tissues from normal eyes for presence of the EBV genome.

METHODS

Ten freshly harvested cadaveric human eyes were dissected into limbal cornea, central cornea, aqueous humor, iris, vitreous humor, and optic nerve. Total cellular DNA preparations were screened for DNA sequences specific to EBV's large internal repeat region. After Southern transfer, polymerase chain reaction products were detected by a 32P-labeled oligonucleotide probe specific to amplified sequences internal to the polymerase chain reaction primers.

RESULTS

Seven of ten eyes from deceased donors yielded a polymerase chain reaction product, indicating presence of EBV genome. In all, 12 (20%) of 60 cadaveric ocular samples contained EBV DNA. Only the optic nerve was consistently negative for EBV DNA.

CONCLUSIONS

Detection of EBV DNA in cadaveric ocular tissues indicates a broad anatomic distribution of this persistent mucosal pathogen. The frequency with which EBV was found at apparently normal ocular sites raises the possibility for viral involvement in disease states, but emphasizes the need for specific criteria to implicate EBV in ocular pathology.

Detection and quantification of virus DNA in plasma of patients with Epstein-Barr virus-associated diseases

Epstein-Barr virus (EBV) causes various diseases, such as infectious mononucleosis (IM), fatal IM, EBV-associated hemophagocytic syndrome (EBVAHS), and chronic active EBV infection (CAEBV). In the present study, cell-free EBV DNA was detected in the plasma of patients with EBV-associated diseases by PCR assay. The patients included 20 patients with IM, 2 patients with fatal IM, 4 patients with EBVAHS, 4 patients with CAEBV, and 38 healthy children (20 EBV seropositive and 18 EBV seronegative). In patients with IM, plasma samples were positive for EBV DNA in all patients (100%) in the acute phase and in 44% of the patients in the convalescent phase, but plasma samples from the 38 healthy control children were negative (0%) for EBV DNA. Quantitative PCR assay revealed that plasma from patients with IM contained the highest amount of virus DNA within 7 days following the onset of disease (mean, 6 x 10(4) copies per ml). The EBV DNA concentration decreased thereafter as the patients recovered. Plasma from patients with fatal IM contained more than 100 times more copies of EBV DNA (3 x 10(7) copies per ml) than plasma from patients with IM. Plasma from patients with the acute phase of EBVAHS contained 10 times more copies of EBV DNA (5 x 10(5) copies per ml) than plasma from IM, and then patients with the number of copies decreased similarly in both groups of patients in the convalescent phase (2 x 10(4) copies per ml). The amount of virus DNA in patients with CAEBV (6 x 10(4) copies per ml) was similar to that noted in patients with IM; however, it became higher (1 x 10(6) copies per ml) when the patients' clinical status deteriorated. These data suggest that the presence of cell-free EBV DNA in plasma is a common phenomenon in patients with EBV-associated diseases. The concentration of EBV DNA in plasma seems to be higher in patients with the more severe clinical categories of EBV diseases.

Detection of herpesvirus-like DNA sequences in Kaposi's sarcoma in patients with and those without HIV infection

BACKGROUND

Herpesvirus-like DNA sequences have recently been found in lesions from patients with Kaposi's sarcoma and the acquired immunodeficiency syndrome (AIDS). It is not known whether these sequences are also present in classic Kaposi's sarcoma or in the Kaposi's sarcoma that occurs in homosexual men who are seronegative for the human immunodeficiency virus (HIV).

METHODS

We analyzed DNA in tissue samples from patients with AIDS-associated Kaposi's sarcoma, patients with classic Kaposi's sarcoma, and HIV-seronegative homosexual men with Kaposi's sarcoma. We also analyzed DNA in samples of uninvolved tissue from these patients and in control tissue from healthy subjects. All samples were tested blindly by polymerase chain reaction (PCR) with specific primers to amplify KS330(233), a herpesvirus-like DNA sequence.

RESULTS

The KS330(233) PCR product was found in 20 of 21 tissue samples (95 percent) from the patients with Kaposi's sarcoma, including 10 of the 11 samples from the patients with AIDS-associated Kaposi's sarcoma, all 6 samples from the patients with classic Kaposi's sarcoma, and all 4 samples from the HIV-negative homosexual men with Kaposi's sarcoma. Only 1 of the 21 control samples (5 percent) was positive (odds ratio, 400; 95 percent confidence interval, 19 to 17,300). Of the 14 samples of uninvolved skin from the patients with Kaposi's sarcoma, 3 were positive for KS330(233). Representative PCR-product sequences were more than 98 percent identical for the three types of Kaposi's sarcoma, suggesting that all three are caused by the same agent.

CONCLUSIONS

The same herpesvirus-like DNA sequences are present in AIDS-associated Kaposi's sarcoma, classic Kaposi's sarcoma, and the Kaposi's sarcoma that occurs in HIV-negative homosexual men. Therefore, this presumably new human herpesvirus is not solely an opportunistic infection in patients with AIDS, and the three forms of Kaposi's sarcoma may be caused by the same infectious agent.