Unidentified bacterial microorganisms entrapped within blood capillary spaces of tissue from different epidemiological types of Kaposi's sarcoma.

Tissue specimens of different epidemiological types of Kaposis sarcoma (KS) from various geographical regions were investigated by transmission electron microscopy. Freshly fixed KS biopsies originated from 9 German patients: 3 classic KS cases, 5 AIDS-associated KS cases, and 1 atypical classic KS case. Additionally, KS autopsy material from the brain of a German AIDS patient was examined. Further biopsies came from 29 Ugandan patients: 16 endemic KS cases and 13 AIDS-associated KS cases. While investigating the ultrastructure, we discovered relatively small-sized bacterial microorganisms within blood capillary spaces of tumor tissue from 5 KS cases of different epidemiological type. The microorganisms often occurred in clusters. They were of coccoid-bacillary form and limited by a wrinkled multilayered cell wall. Many of them were encapsulated. They were not observed outside of the capillary lumen. The bacterial structures were often seen attached to capillary endothelial cells, which sometimes showed blistering into the capillary lumen. The observed bacterial microorganisms obviously represented agents of a bloodstream infection and must have been entrapped and accumulated within capillary spaces of KS tissue. The bacteria, which had an almost identical morphology in all 5 KS cases, could not be identified. If they are of pathogenic significance, it remains unknown.     

Occurrence of tubuloreticular structures and intracisternal paracrystalline inclusions in endothelial cells of tissue from different epidemiological types of Kaposi's sarcoma

Biopsied tissue specimens from 40 cases of classic, atypical classic, endemic, and AIDS-associated Kaposi's sarcoma (KS) were investigated by electron microscopy. To search for ultrastructural differences between non-AIDS-associated KS and AIDS-associated KS, the occurrence of the following 2 ultrastructural abnormalities of the rough-surfaced endoplasmic reticulum in KS cells was evaluated semi-quantitatively: tubuloreticular structures (TRS) and intracisternal paracrystalline inclusions (IPI). These peculiar structures were found in 23 of the 40 KS cases. Two types of TRS could be distinguished: loose TRS (LTRS) and compact ones (CTRS). LTRS were observed in endothelial cells of tissue from all the different epidemiological types of KS. CTRS were confined to AIDS-associated KS. IPI were present in endothelial tumor cells of only 3 non-AIDS-associated KS cases. The study shows that in cells of KS tissue only CTRS, but not LTRS, are an ultrastructural marker for AIDS-associated KS.     

Kaposi's sarcoma occurring during short-term dialysis: report of two cases

Kaposi's sarcoma (KS) appears to develop in association with kidney transplantation, but unlikely with dialysis. We report two cases of classic KS that occurred in patients receiving short-term (less than 3 yr) dialysis. They have been suffering from chronic renal failure due to tuberculosis and diabetes mellitus, respectively. Several to multiple, reddened-violaceous patches, plaques and nodules were found on the hand and the lower extremities. Laboratory studies showed no evidence suggesting immunosuppressed state and there was no history of taking immunosuppressive agents. The biopsies of the two cases revealed proliferation of spindle-shaped cells focally arranged in bundles and multiple dilated vascular spaces outlined by an attenuated endothelium with intravascular and extravasated erythrocytes. The specimens expressed positivity with CD34 antigen. Human herpesvirus 8 (Kaposi's sarcoma-associated herpesvirus) was detected in one case by polymerase chain reaction method.     

Ultrastructural Changes in the Capillary Bed of Human Pituitary Tumors

The fine structure of the capillary bed of the anterior pituitary has been studied in 19 cases of pituitary tumor and 1 autopsy specimen. Tumor specimens were less well vascularized than the autopsy specimen. Endothelial cells within tumor specimens were often observed with swollen portions of cytoplasm, or cytoplasmic blebs, projecting into the capillary lumen. Blebbing, in many cases, nearly obstructed the capillary lumen and was most often associated with endothelial cells that had an electron-lucent cytoplasm, in contrast to endothelial cells that had an electron-dense cytoplasm, even within the same capillary profile. Compared with electron-lucent cells, electron-dense cells contained more endothelial filaments. Also observed were capillaries that had apparently broken apart releasing their contents into the pericapillary space and shrunken remnants of capillaries. A number of abnormal features were observed in the pericapillary spaces—ie, disruption of the parenchymal-pericapillary interface, disorganization of basal laminae, increased amounts of plasma proteins and cellular debris within the space and, ultimately, complete loss of the normal limits and characteristics of the space. Similar, though less pronounced, changes were observed in the autopsy specimen. The hypothesis is advanced that changes observed in the capillary bed are, in large part, a result of tumor growth, which increasingly disrupts tissue organization at the parenchymal-pericapillary interface and, because the tumor mass in the sella turcica can only enlarge upwards, compresses the pituitary stalk and portal veins. The result is ischemia and eventual necrosis. The observations are then in good agreement with recent reports on changes in capillary fine structure associated with ischemia.     

Distribution of basement membrane-associated heparan sulfate proteoglycan in idiopathic and AIDS-associated Kaposi's sarcoma

In the present study we analyzed the immunohistochemical distribution of different major basement membrane (BM) components with special emphasis on the BM-associated heparan sulfate proteoglycan (HSPG) in early and late stages of Kaposi's sarcoma (KS), both of idiopathic and AIDS-associated origin. In early KS all BM components tested were found surrounding the small clefts of tumour vessels. Heparan sulfate proteoglycan showed the weakest and often fragmented pattern of staining. In the late, nodular sarcomatous form of KS individual tumour cells were surrounded by a BM composed of collagen IV, laminin and fibronectin, while heparan sulfate proteoglycan was not detectable in most cases. Neither between idiopathic and AIDS-associated KS nor between cutaneous and visceral lesions were significant differences in the staining pattern. Our findings of a rather selective expression of various BM-components and the known distribution in normal blood and lymphatic capillaries raises the hypothesis that KS-cells may be derived from cells of lymphaticovenous differentiation.     

Histology of early lesions of AIDS-associated Kaposi's sarcoma

The original cutaneous biopsy specimens of 93 patients who presented themselves to the Memorial Sloan-Kettering Cancer Center with acquired immunodeficiency syndrome (AIDS)-related Kaposi's sarcoma (KS) were systematically reviewed for 23 histologic variables. KS was the initial manifestation of AIDS in all of the patients. The vast majority of patients presented with plaque histology of KS. Early lesions of KS were characterized by the presence of dilated vascular spaces haphazardly arranged in the biopsy specimen, a sparse inflammatory cell infiltrate composed of lymphocytes (usually without plasma cells), and aggregates of cuboidal cells with the appearance of epithelioid cells. Individually necrotic tumor cells were present in nearly every case. Spindle cells arranged in fascicles or nodules were seen in a minority of cases. These data provide an overview of the different histologic patterns seen in initial lesions of AIDS-associated KS and may lead to better understanding of the pathogenesis of this tumor.     

Molecular epidemiology of human herpesvirus 8 variants in Kaposi's sarcoma from Iranian patients

Kaposi's sarcoma (KS) is a rare cancer in Iran and there is no epidemiological and molecular information about HHV-8 variants circulating among the Iranian population. In this study HHV-8 sequences have been analyzed in 43 cutaneous KS biopsies from Iranian patients mainly affected by classic KS. DNA samples were subjected to PCR amplification of HHV-8 ORF26, T0.7 and K1 followed by direct nucleotide sequencing and phylogenetic analysis. The analysis of ORF26 showed that 30 (69.8%) and 13 (30.2%) samples belonged to subtypes A/C and K, respectively. In general, the clustering of HHV-8 T0.7 variants paralleled that of ORF26. Genotyping of K1 sequences showed that the majority of samples (39 out of 41) fall into the large C clade with only 2 belonging to the A clade. In conclusion, HHV-8 variants identified among classic Iranian KS are largely related to Eurasian genotypes previously identified in KS from Mediterranean, Middle East, and East Asian regions.     

The expression of endothelial cell surface antigens by AIDS-associated Kaposi''s sarcoma. Evidence for a vascular endothelial cell origin.

The authors investigated 19 cases of Kaposi's sarcoma (KS) obtained from patients with the acquired immune deficiency syndrome (AIDS) for their expression of Factor VIII-related antigen (FVIIIRAg), HLA-DR (Ia) antigens, OKM1, and three distinctive vascular, but not lymphatic, endothelial-cell-associated antigens, E92, OKM5, and HCl. Antigen expression was demonstrated by immunoperoxidase staining of cryostat sections. FVIIIRAg is strongly expressed by the cells lining the vascular spaces (VCs) but is absent, weakly or focally, and variably expressed by the spindle cell (SC) component of KS. The VC component of each KS lesion examined strongly expressed E92, moderately expressed HCl, and weakly expressed OKM5. In contrast, the entire SC component of each KS lesion studied strongly expressed E92 and OKM5 and weakly expressed HCl. Neither the VCs nor the SCs expressed OKM1. These studies provide strong and compelling evidence for the vascular endothelial cell histogenesis of both the vascular and spindle cell components of KS, demonstrate the intertumor and intratumor phenotypic heterogeneity of KS, and suggest that monoclonal antibodies OKM5 and anti-E92 are the best currently available immunohistochemical markers for identifying the spindle cell component of AIDS-associated KS in cryostat sections.     

The extracellular matrix in oral Kaposi sarcoma (AIDS): the immunohistochemical distribution of collagens type IV,V, VI,of procollagens type I and III,of laminin and of undulin

Summary Twelve oral AIDS-associated Kaposi sarcomas (KS) were studied for the distribution of extracellular matrix components using indirect immunofluorescence. Staining for basement membrane (BM) components revealed two distinct patterns of distribution: a delicate and partly fragmented lining of BMs around sinusoid-like vascular spaces or an occasional diffuse interstitial fluorescence in the tumour stroma; or an irregular broad rim of fluorescence in walls of larger blood vessels. These findings support a vascular cell origin of the endothelial- and spindle cell component in KS. The tumour stroma was almost completely negative for collagen type V and undulin, whereas an intensive fluorescence was noted for procollagens type I, III and collagen type VI. In areas adjacent to KS a loss of procollagens type I and III, collagens type V, VI and undulin was noted. An intimal sheath of collagen type V was usually absent from blood vessels of the tumour or the peritumourous connective tissue. Immunohistochemical findings indicate that the preexisting interstitial connective tissue matrix is destroyed during tumour invasion and that subsequently procollagens type I, III and collagen type VI are synthesized de novo by cells of the tumour stroma.This study was supported by the Deutsche Forschungsgemeinschaft (Be 1017/1-2)     

Immunihistochemical detection of Bcl-2 in AIDS-associated and classical Kaposi's sarcoma.

Kaposi''s Sarcoma (KS) is an angioproliferative disease that is characterized by proliferation of spindle-shaped cells predominantly of vascular endothelial cell origin, neoangiogenesis, inflammatory cell infiltration, and edema. Although the lesions of classical KS and AIDS-associated KS (AIDS-KS) share common histological features, AIDS-KS occurs at a markedly higher frequency with a more aggressive clinical course. Immunohistochemical analyses of 26 evolutionarily staged AIDS-KS lesions derived from HIV-infected patients demonstrate significant cytoplasmic levels of Bcl-2, a protooncogene known to prolong cellular viability and to antagonize apoptosis. Bcl-2 expression increases as the pathological stage of KS advances. Immunohistochemical analyses of classical KS lesions demonstrate prevalent expression of Bcl-2 as well, indicating that upregulation of Bcl-2 may be important in the pathogenesis of both classical and AIDS-associated KS. Coexpression of Bcl-2 and factor VIII-related antigen in spindle-shaped cells present within KS lesions suggests that Bcl-2 is upregulated within the vascular endothelial spindle-shaped cells of KS. The consequences of upregulated Bcl-2 expression within KS lesions may be prolonged spindle cell viability which, when coupled with dysregulated cellular proliferation due in part to synergistic activities of inflammatory and angiogenic cytokines and HIV-1 Tat protein, may result in the maintenance, growth, and progression of KS.     

Molecular epidemiology of Kaposi's sarcoma-associated herpesvirus/human herpesvirus 8 strains from Russian patients with classic, posttransplant, and AIDS-associated Kaposi's sarcoma

We report the molecular characterization of 38 new Kaposi's sarcoma-associated herpesvirus (KSHV) strains from Russian patients with either classic (25 cases), epidemic/AIDS-associated (7 cases), or posttransplant/immunosuppressed patients (6 cases), or Kaposi's sarcoma (KS). While a complete sequence of the K1 gene (870 bp) was obtained from 30 strains, only partial sequences of the hypervariable regions VR1 (372 bp) and/or VR2 (381 bp) of the K1 gene were obtained from eight strains of KS paraffin blocks. Sequence comparison and phylogenetic studies indicate that the novel KSHV strains belong to either the A subtype (28 cases) or the C subtype (10 cases). Within the 28 strains of A subtype, 24 (86%) belong to the large A' subgroup, mostly A1 and A1' clades, and 4 belong to the A" subgroup, mostly A3 clade. Within the 10 strains of subtype C, 4 were of C' subgroup, and 6 of the C". Some molecular variants of subtype A' were observed, with 3 strains exhibiting an insertion of a single amino acid at the position 65 and 2 strains (both from AIDS-KS) with an unique deletion of 17 amino acids in the VR2 region. Polymerase chain reaction-based subtyping of the K14.1 genomic region indicated that most (23/32) of the novel strains belonged to the P subtype. The results indicate that despite a wide genetic diversity of A and C K1 subtypes of KSHV strains present in Russia, most are closely related and belong to the A1 or A1' molecular clades suggesting a common origin. This study also expands the data regarding the absence of any correlation between a K1 molecular subtype and a specific KS type (classic, epidemic, or posttransplant), as well as between the K1 and K14.1 molecular subtypes.     

Detection of several types of human papilloma viruses in AIDS-associated Kaposi's sarcoma

Epidemiological studies indicate that acquired immunodeficiency syndrome (AIDS)-associated Kaposi's sarcoma (KS) may be caused by an infectious, preferentially sexually transmitted agent. Infections with human papilloma viruses are common, sexually transmitted diseases occurring frequently in homosexual men, who are also the main risk group of developing KS. In order to evaluate the possible role of HPV in the development of KS, 24 cutaneous AIDS-associated Kaposi's sarcomas were investigated by the polymerase chain reaction (PCR) and by in situ hybridization for the presence of human papilloma viruses (HPV). HPV DNA sequences were detected in 5 of 24 KS specimens, in 4 of 13 normal skin specimens from AIDS patients withoutKS and in 5 of 14 skin specimens of HIV-seronegative patients. For the first time, HPV types 6 and 33 were detected by PCR in KS. A higher proportion of HPV types 16/18 was found in AIDS-associated KS specimens, whereas HPV type 33 was seen more often in normal skin specimens of the control group. Apart from the known HPV types 16/18 described in KS, this study demonstrates also the presence of HPV 6 and 33 in this condition. © 1995 Wiley-Liss, Inc.     

Differential diagnosis of Kaposi's sarcoma

The biopsies of all lesions clinically thought to be suspicious for Kaposi's sarcoma (KS) were reviewed over a 15-month period. A diagnosis of KS was made in 40 of 106 biopsies (38%). The cases in which a diagnosis other than KS was made included dermatofibroma, hemangioma, and scar. This second group comprised 59 of 106 cases (56%). A third group included some lesions that had an atypical vascular proliferation, but in which the changes were insufficient for a definite diagnosis of KS. The presence of abnormally shaped vessels, especially those classified as irregular, was the best single criterion to diagnose KS in its early stages. In later stages, the neoplasm assumes a nodular configuration with typical, slitlike vascular channels. At the periphery of such nodules dilated, irregularly shaped vessels similar to those of the early lesions are often seen. The histologic features which help in the diagnosis of KS from other histologic entities are reviewed.     

Detection of KSHV in transbronchial biopsies in patients with Kaposi sarcoma.

In transbronchial biopsies, Kaposi sarcoma (KS) is difficult to correctly diagnose by H&E staining due to the inherent vascular nature of the lung tissue, coupled with the subtle nature of the changes in early KS. Since KS-associated herpesvirus (KSHV) has been found in all clinical forms of KS, the detection of KSHV genomic DNA sequences and/or viral products may be helpful in the diagnosis of pulmonary KS. From their files during the past 10 years, the authors identified ten HIV/AIDS patients who were positive for KS in transbronchial biopsies and four HIV/AIDS patients with KS-negative transbronchial biopsies. Immunohistochemistry with antibodies against the latency-associated nuclear antigen (LANA-1 or LNA) of KSHV was performed. Nested polymerase chain reactions (PCR) with KSHV ORF-K1 or -K9 were performed in all cases, and the KSHV sequences were detected in 9/10 (90%) KS cases. Immunohistochemical analysis was able to detect 4/10 (40%) cases. One case was negative by both PCR and immunohistochemistry. Of the five KS cases that were not diagnosed definitively ("consistent with" or "suspicious for"), two were confirmed by both immunoreactivity and PCR. One of the negative control cases was positive for KSHV by PCR but not by immunohistochemistry. The patient was thereafter found to have a clinical history of pulmonary KS at another hospital. In conclusion, in transbronchial biopsies of the lung suspicious for KS, PCR is the most sensitive technique available for clinical diagnosis of KS. Immunohistochemistry analysis might be helpful in difficult pulmonary KS cases.     

Basement membrane in Kaposi's sarcoma: an immunohistochemical and ultrastructural study

Basement membranes were investigated in early angiomatous and late sarcomatoid stages of Kaposi's sarcoma (KS). Seven frozen skin biopsies of KS from five elderly Mediterranean people and one renal allograft recipient were labelled, using an immunoperoxidase technique, for basement membrane-specific macromolecules, laminin and type IV-collagen. Twenty-seven other frozen cutaneous lesions including haemangio and lymphangiosarcomas, benign vascular tumours, and various epithelial, melanocytic, fibrohistiocytic, fibrosarcomatous and muscular tumours were processed in the same way. In addition an ultrastructural study was performed in two cases of KS, one haemangiosarcoma and one lymphangiosarcoma. Intense labelling was observed for both type IV-collagen and laminin, which appeared closely co-distributed, in all areas of KS. Staining pattern was often regular and continuous around neoplastic vessels in early lesions of KS, as in benign vascular lesions, whereas in late nodular lesions large amounts of basement membrane components were present in intercellular spaces between densely aggregated spindle cells. In contrast, ultrastructural examination disclosed early disruption of basement membranes around neoplastic vessels, and occasional fragments of external lamina were seen at the interface between KS spindle cells and collagen. Similar results were obtained in angiosarcomas both by immunohistological and ultrastructural study. In the comparative group, laminin and type IV-collagen were present, in significantly fewer quantities and in various distinctive patterns, in epithelial, melanocytic, fibrohistiocytic and muscular tumors. This study deals with basement membrane modifications in early and late lesions of KS and provides further evidence in favour of the endothelial nature of the spindle cells of KS.     

Changes in the small biliary passages in the hepatic localization of Hodgkin's disease

Summary The authors have investigated the behaviour of the small biliary passages in the liver biopsies of six patients suffering from untreated Hodgkin's disease with hepatic localization. No obstruction of the major bile ducts was demonstrated in any patient. Three of the patients were anicteric, while the three others presented with jaundice. In the first three cases typical Hodgkin's granulation tissue appears to be limited to portal tracts and collagen reaction is virtually absent. The three cases with cholestasis showed granulomatous tissue associated with heavy connective tissue rearrangement invading and dissociating the lobular structure. They also show a conspicuous bile-duct proliferation, which is not observed in the three anicteric patients. In these latter cases, however, the small bile ducts running within or near the granulomatous tissue present various morphologic changes, including basal membrane thickening, dilation or constriction of the lumen and alterations of the biliary epithelial lining. Complete disappearance of the bile duct may occur.The authors are indebted to Professor Valeer J. Desmet, Chief of the Department of Pathology II, Leuven, Belgium, for his appreciation and valuable suggestions     

Strong expression of kinase insert domain-containing receptor, a vascular permeability factor/vascular endothelial growth factor receptor in AIDS-associated Kaposi's sarcoma and cutaneous angiosarcoma.

Vascular permeability factor (VPF), also known as vascular endothelial growth factor (VEGF), plays an important role in the angiogenesis associated with the growth of many human and animal tumors. VPF/VEGF stimulates endothelial cell growth and increases microvascular permeability by interacting with two endothelial cell tyrosine kinase receptors, KDR and flt-1. We studied 16 cases of AIDS-associated Kaposi's sarcoma (KS), 2 cases of cutaneous angiosarcoma, and 6 cases of capillary hemangioma by in situ hybridization for expression of VPF/VEGF, KDR, and flt-1 mRNAs. We also performed immunohistochemical staining for VPF/VEGF protein in 15 cases. Tumor cells in KS and angiosarcoma strongly expressed KDR but not flt-1 mRNA. Endothelial cells in small stromal vessels in and around these tumors strongly expressed both KDR and flt-1 mRNAs. Tumor cells expressed VPF/VEGF mRNA strongly in only one case of KS, adjacent to an area of necrosis. This was also the only case in which the tumor cells stained substantially for VPF/VEGF protein. VPF/VEGF mRNA and protein were, however, strongly expressed by squamous epithelium in areas of hyperplasia and near areas of ulceration overlying tumors. VPF/VEGF mRNA was also expressed focally at lower levels by infiltrating inflammatory cells, probably macrophages. The strong expression of both KDR and flt-1 in small stromal vessels in and around tumors suggests that VPF/VEGF may be an important regulator of the edema and angiogenesis seen in these tumors. The strong expression of KDR by tumor cells in KS and angiosarcoma implies that VPF/VEGF may also have a direct effect on tumor cells. Tumor cells in four of six capillary hemangiomas strongly expressed both KDR and flt-1 mRNAs in contrast to the high level expression of only KDR observed in the malignant vascular tumors studied. Neither VPF/VEGF mRNA or protein were strongly expressed in capillary hemangiomas. VPF/VEGF and its receptors may play an important but as yet incompletely understood role in the pathogenesis of both benign and malignant vascular tumors.     

Human bcl-2 expression, cleaved caspase-3, and KSHV LANA-1 in Kaposi sarcoma lesions

We aimed to evaluate the frequency of Kaposi sarcoma (KS)-associated herpesvirus (KSHV) infection in KS lesions in patients from Brazil. In addition, expression of human bcl-2, cleaved caspase-3, and KSHV latency-associated nuclear antigen (LANA)-1 in tumors was evaluated using immunohistochemical analysis. We studied 64 KS cases, classified as follows: classical, 20 (31%); iatrogenic, 2 (3%); AIDS-associated, 25 (39%); and not otherwise specified (lack of information about HIV status), 17 (27%). KSHV was detected by polymerase chain reaction (PCR) in 61 cases (95%); 40 cases (63%) were KSHV+ by PCR and immunohistochemical analysis for LANA-1. Immunoexpression of bcl-2 was detected in 47 cases (73%). Only a few cells in 15 cases (23%) of KS had demonstrable immunostaining for cleaved caspase-3. These results further support the association of KSHV with all KS forms. Cleaved caspase-3 in KS tumors was infrequent, which may reflect the inhibition of apoptosis owing to bcl-2 overexpression observed in the majority of KS tumors.     

Human herpesvirus 8 in Kaposi's sarcoma and Kaposi's sarcoma-mimicking vascular tumors.

Kaposi''s sarcoma (KS) had been a rare and unusual vascular tumor until a recent epidemic of a disseminated and fulminant form of KS in AIDS patients. Infectious agents have been suspected of causing KS, and recently partial genomic DNA sequences of human herpesvirus 8 (HHV8) have been identified in AIDS-associated KS lesions. Since then, genomic DNA sequences of HHV8 have been isolated in other forms of KS. Although the partial genomic DNA sequence of HHV8 was reported to be, if rare, identified in vascular tumors other than Kaposi''s sarcoma (KS), the presence of HHV8 in a very large fraction of KS indicates that detection of HHV8 by PCR is a useful auxiliary tool in differentiating KS from other KS-mimicking vascular tumors. We examined whether the 233-bp segment of the viral DNA was detected in Korean patients with KS and other KS-mimicking vascular tumors. HHV8 sequences were identified in all of nine classic type of KS but not in three epithelioid hemangioendotheliomas and seven angiosarcomas. Our results confirm the relatively restricted distribution of HHV8 and also argue against the likelihood of secondary colonization of KS cells by HHV8.     

Detection of antibodies against viral capsid proteins of human herpesvirus 8 in AIDS-associated Kaposi’s sarcoma

 Sequences of a new herpesvirus with homology to gammaherpesvirinae were recently identified in AIDS-associated Kaposi’s sarcoma (KS). Subsequently this novel virus, called KS-associated virus (KSHV) or human herpesvirus (HHV) 8 was detected in classical KS and AIDS-associated body cavity based lymphomas by polymerase chain reaction. In this report major and minor capsid proteins of HHV-8 were molecularly cloned and produced as recombinant proteins in Escherichia coli. Sera from 69 HIV-1 infected patients with KS, 30 HIV-1 infected patients without KS and 106 control individuals were tested by enzyme-linked immunosorbent assay for anti-HHV-8 capsid IgM and IgG antibodies. Sera from four patients were tested over periods ranging from 18 months to 6 years. IgG antibodies directed against HHV-8 capsid antigens were detected in patients with AIDS-associated KS and in some AIDS patients without KS. Seroconversion with IgM and IgG antibodies directed against HHV-8 capsid proteins occurred more than 1 year prior to diagnosis of KS. In a considerable portion of KS patients no IgM or IgG antibodies against HHV-8 capsid proteins were detected. In these patients there was an inverse relationship between antibodies against HHV-8orf26 and the CD4/CD8 ratio, suggesting that the inconsistency of anti-HHV-8orf26 antibodies is due at least partly to an impaired immune response. No reactivity against HHV-8 capsid antigens was detected in the vast majority of sera from HIV-negative control individuals. Our findings indicate that a specific humoral immune response against capsid proteins is raised in HHV-8 infected individuals, and that anti-capsid antibodies can be used to diagnose HHV-8 infection. The correlation between occurrence of anti-HHV-8 antibodies and KS supports the hypothesis of a causative role of HHV-8. Received: 3 August 1996 / Accepted: 28 November 1996