Selective accumulation of virus-specific CD8+ T cells with unique homing phenotype within the human bone marrow

The bone marrow plays a unique role within the immune system. We compared the phenotype and function of virus-specific CD8(+) T cells from matched samples of human peripheral blood and bone marrow. Analysis of virus-specific memory CD8(+) T cells showed widely divergent partition of antigen-specific populations between blood and bone marrow. T cells specific for Epstein-Barr virus (EBV) lytic antigens were enriched 3-fold in marrow compared with blood, whereas the response to EBV latent epitopes was equivalent between the 2 compartments. No difference in EBV viral load or expression of the EBV lytic protein was observed between blood and bone marrow. In direct contrast, although cytomegalo-virus (CMV)-specific T cells were the largest virus-specific population within peripheral blood, they were reduced by 60% within marrow. Bone marrow T cells were found to exhibit a unique CCR5(+)CXCR6(+)CXCR3(-) homing phenotype which has not been observed on T cells from other secondary lymphoid organs or peripheral organs. Expression of CCR5 and CXCR6 was higher on EBV-specific T cells within peripheral blood compared with CMV-specific populations. These observations identify a novel bone marrow homing phenotype for CD8(+) memory T cells, which necessitates a reevaluation of the magnitude of antigen-specific populations within the lymphoid system.     

Cytogenetic studies of bone marrow fibroblasts cultured from patients with myelofibrosis and myeloid metaplasia.

Cytogenetic studies of bone marrow fibroblasts and blood cells from peripheral blood or bone marrow were performed in 19 patients with myelofibrosis with myeloid metaplasia (group 1), nine patients with other myeloproliferative syndromes without myelofibrosis (group 2), and 12 patients with anaemia secondary to iron deficiency or chronic inflammatory disease (group 3). Clonal cell populations with abnormal karyotypes were seen in the bone marrow or blood in five of 14 (36%) group 1 patients, one of nine (11%) group 2 patients and none (0%) of the group 3 patients. Abnormal karyotypes of bone marrow fibroblasts were found in three of 16 (19%) of patients of group 1, and in two of nine (22%) and two of 12 (17%) patients each of groups 2 and 3, respectively. Since abnormal karyotypes can be found in bone marrow fibroblasts cultured from normal subjects, and since the abnormalities seen in the bone marrow fibroblasts differed from those found in bone marrow or blood cells, it is suggested that abnormal karyotypes found in bone marrow fibroblasts cultured from patients with primary myelofibrosis do not necessarily reflect neoplasia. The results of this study are compatible with the widely accepted hypothesis that in patients presenting with 'primary' myelofibrosis, the fibrosis is a secondary reactive process.     

The Italian Consensus Conference on Diagnostic Criteria for Myelofibrosis with Myeloid Metaplasia

The purpose of this work was to develop a definition of myelofibrosis with myeloid metaplasia (MMM) using diagnostic criteria that would remain valid within the set of patients with chronic myeloproliferative disorders or myelodysplastic syndromes. A list of 12 names for the disease and 37 diagnostic criteria were proposed to a Consensus Panel of 12 Italian experts who ranked them in order so as to identify a core set of criteria. The Panel was then asked to score the diagnosis of 46 patient profiles as appropriate or not appropriate for MMM. Using the experts' consensus as the gold standard, the performance of 90 possible definitions of the disease obtained through the core set was evaluated. 'Myelofibrosis with myeloid metaplasia' ranked as the preferred name of the disease. Necessary criteria consisted of 'diffuse bone marrow fibrosis' and 'absence of Philadelphia chromosome or BCR-ABL rearrangement in peripheral blood cells'. The six optional criteria in the core set consisted of: splenomegaly of any grade; anisopoikilocytosis with tear-drop erythrocytes; the presence of circulating immature myeloid cells; the presence of circulating erythroblasts: the presence of clusters of megakaryoblasts and anomalous megakaryocytes in bone marrow sections; myeloid metaplasia. The definition of the disease with the highest final score was as follows: necessary criteria plus any other two criteria when splenomegaly is present or any four when splenomegaly is absent. The use of this definition will help to standardize the conduct and reporting of clinical studies and should help practitioners in clinical practice.     

Reappearance of t(12;17)-positive primary myelofibrosis following Ph+ CML cell reduction by imatinib

A 67-years old woman was referred to our hospital in October 1992 with thrombocytopenia and splenomegaly. A bone marrow biopsy revealed decreased cellularity, with moderately increased reticulin fibrosis and discrete dysmorphic megakaryocytes but no signs of dysplasia in the erythroid or the myeloid lineages. The karyotype of the bone marrow cells was t(12;17) (q24;q11). She was diagnosed as having agnogenic myeloid metaplasia. The patient received only blood transfusions until November 1998 when leukocytosis with immature cells started to appear. The bone marrow aspiration analysis showed increased cellularity and chromosomal analysis demonstrated the presence of t(9;22) (q34;q11) without any t(12;17) (q24;q11) abnormality. Because IFN therapy and oral administration of hydroxyurea did not show any cytological effect, administration of imatinib mesylate was started from December 2001. The Ph-positive cells as demonstrated by the FISH method had decreased to 7% by April 2003. But the t(12;17)(q24;q11) positive clones, which were observed on the first admission, again appeared in the peripheral blood, whereas Ph clones were detected in only one out of 24 cells examined. During the course of treatment with imatinib mesylate for chronic myelogenous leukemia which developed from agnogenic myeloid metaplasia accompanied with t(12;17)(q24;q11) translocation, the co-existence of two clones derived from, possibly, stem cells was identified.     

Clonal granulocytes and bone marrow cells in the cellular phase of agnogenic myeloid metaplasia.

Myelofibrosis with myeloid metaplasia (MMM) belongs to the group of myeloproliferative syndromes. It is characterized by a sustained proliferation of megakaryocytes and increased medullary reticulin fibers. Until now the cellular phase at onset of the disease has not been analyzed for clonality of the hematopoietic cells. In this study we used X-linked restriction length polymorphism (RFLP) analysis to investigate the clonality of granulocytes and bone marrow cells from the cellular phase and advanced stages of the disease. In each of 12 heterozygous females, monoclonality of granulocytes or total bone marrow cells could be demonstrated. These results show that the cellular phase represents a monoclonal, and hence a probably neoplastic, proliferation of a pluripotent stem cell. The monoclonality of granulocytes present at the onset of disease should allow analysis of DNA of these easily accessible peripheral cells for the detection of specific clonal aberrations.     

Involvement of subchondral bone marrow in rheumatoid arthritis: lymphoid neogenesis and in situ relationship to subchondral bone marrow osteoclast recruitment

OBJECTIVE: To evaluate the presence and immunohistochemical characteristics of subchondral bone marrow inflammatory infiltrate in rheumatoid arthritis (RA) and to determine the in situ relationship between marrow inflammation and osteoclast recruitment. METHODS: Bone samples and paired synovia from 8 RA patients undergoing joint surgery were analyzed by immunohistochemistry and in situ hybridization for specific lymphoid neogenetic features, such as T and B cell composition, follicular dendritic cell (FDC) networks, peripheral lymph node addressin (PNAd)-positive high endothelial venules, and lymphoid chemokine expression. Osteoclasts were identified as multinucleated tartrate-resistant acid phosphatase (TRAP)-positive and cathepsin K-positive cells adherent to the bone surface. RESULTS: An inflammatory infiltrate with perivascular aggregates of variable size was detected in 7 (87.5%) of 8 synovial samples and in paired bone samples. Lymphoid neogenetic features typical of rheumatoid synovium were also recognized in the bone marrow. PNAd+ blood vessels were found in 4 of 8 patients, CD21+ FDC networks in 2 patients, CXCL13+ cells in 5 patients, and CCL21+ cells in 6 patients. TRAP-positive and cathepsin K-positive osteoclasts were identified on both the synovial and marrow sides of the bone surface. Bone marrow samples showing a higher degree of inflammation were characterized by a significantly increased number of osteoclasts adherent to the subchondral bone. CONCLUSION: Our data demonstrate that lymphoid aggregates with lymphoid neogenetic features are detectable on the subchondral side of the joint in established RA. Moreover, the local inflammation/aggregation process appears to be related to osteoclast differentiation on the marrow side of subchondral bone, supporting a functional role of the bone compartment in local damage.     

Hypopituitarism and hematological abnormalities mimicking myelodysplastic syndrome. Report of four cases

Insufficiency of the pituitary gland and hematological abnormalities may coexist in the context of two syndromes. In the course of some hematopoietic neoplasms, particularly acute leukemias, the pituitary insufficiency may be caused by destruction of the gland either by direct neoplastic infiltration or occlusion of vessels. Alternatively, thy pituitary dysfunction may be associated with but not caused by hematological abnormalities, usually mild peripheral cytopenias. We present four cases of the latter type (1. M/33, pituitary tumor, hypogonadism, hyperprolactinemia, anemia, mild leukopenia with leukocytosis, 2. F/54, pituitary tumor, hyperprolactinemia, thyreotropic and corticotropic insufficiency, anemia, thrombocytopenia, mild neutropenia, 3. F/27, pituitary tumor, diabetes insipidus, hypogonadism, sideropenic anemia, leukopenia, thrombocytopenia, 4. M/24, primary multihormonal insufficiency of the anterior portion of the pituitary gland, neutropenia, microcytosis). Trephine and aspiration bone marrow biopsies revealed topographic and cytological abnormalities partially resembling these found in myelodysplastic syndromes (MDS). Bone marrow cellularity varied markedly between and within the cases, and in three patients abnormal aplastic areas were found. The percentage of hematopoietic stem cells (CD34+) was low in three cases and normal in one case. Pituitary dysfunction may be associated with hematological abnormalities simulating MDS, but showing different, less aggressive clinical course. The proliferative potential of hematopoietic cells is low, the peripheral blood abnormalities are stable, and no patient developed acute leukemia. Detailed bone marrow examination, including the trephine bone marrow biopsy, is useful in differentiation with true MDS, which was also reported in the literature in the patients with pituitary insufficiency.     

Loss of Rh antigen associated with acquired Rh antibodies and a chromosome translocation in a patient with myeloid metaplasia.

Myeloid metaplasia is a clonal disease of the marrow pluripotent stem cell in which a constant cytogenetic abnormality could result in an altered antigenic characteristic of hematopoietic cells. A 57-yr-old man who had acquired myeloid metaplasia at age 37 was noted to be Rh negative, although blood typing at age 33 was Rh positive. His erythrocytes reacted with anti-c and anti-e, typical for an Rh negative individual. Analysis of 11 metaphase bone marrow cells by G-banding revealed 46 chromosomes with a consistent anomaly in 100% of cells involving chromosomes 1 and 13. The changes were consistent with a reciprocal translocation, with break points at approximately 1p32 and 13q22, although break points at 1p13 and 13q14 were also possible. Since previous cytogenetic data have localized the Rh gene to the short arm of chromosome 1, our data indicate the Rh gene lies within the segment u13 leads to 1p32. Serum of this patient had circulating anti-D and anti-C antibodies. Consequently, this patient lost immunologic tolerance to a "self antigen" after losing the ability to express this antigen.     

Increasing Flt3L availability alters composition of a novel bone marrow lymphoid progenitor compartment

We have recently described a CD19(-) B220(+)CD117(low) bone marrow subpopulation with B, T, and myeloid developmental potential, which we have called "early progenitors with lymphoid and myeloid potential" or EPLM. These cells also expressed Fms-like tyrosine kinase 3, Flt3, or CD135. Treatment of mice with the corresponding ligand, Flt3L, showed a 50-fold increase in EPLM. In addition to the expected increase in dendritic cell numbers, Flt3L treatment had a reversible inhibitory effect on B lymphopoiesis. Limiting dilution analysis of sorted EPLM from Flt3L-treated mice showed that B-lymphocyte progenitor activity was reduced 20-fold, but that myeloid and T-cell progenitor activity was largely preserved. EPLM from treated mice transiently reconstituted the thymus and bone marrow of recipient mice, generating cohorts of functional T and B cells in peripheral lymphoid organs. Thus, Flt3L treatment results in a dramatic increase in a novel bone marrow cell with lymphoid and myeloid progenitor activity.     

Multiple lymphoid nodules in bone marrow have the same clonality as underlying myelodysplastic syndrome recognized with fluorescent in situ hybridization technique

Benign nodular lymphoid lesions are not rare in the bone marrow of patients with myelodysplastic syndrome (MDS). Herein, we report a case of MDS with clonal lymphoid aggregates in the bone marrow but without evidence of systemic lymphoma. The case of a 71-year-old man was evaluated for cytopenia. His bone marrow was initially hypocellular, with 10% blasts and a few small lymphoid aggregates. The diagnosis of refractory anemia with excess blasts was made. The disease progressed gradually, and he received erythropoietin and granulocyte colony-stimulating factor for a short time. Forty-two months later, acute leukemia (M1) developed, with 60% to 70% blasts in the bone marrow. The bone marrow also showed large aggregates of lymphocytes. Immunohistochemical study of these cells in the nodular lesions showed 50% CD3+ and 50% CD20+. Cytogenetic and molecular genetic studies revealed monosomy 7 and T- and B-cell clonal gene rearrangement. Fluorescent in situ hybridization study with centromere-specific probes of a bone marrow specimen showed monosomy 7 in both nodular lymphoid lesions and surrounding bone marrow cells, indicating that both processes originated from the same abnormal pluripotential progenitor. Am. J. Hematol. 59:252–257, 1998. © 1998 Wiley-Liss, Inc.     

Prognostic Significance of Bone-Marrow Patterns in Chronic Lymphocytic Leukaemia

S ummary . Bone-marrow biopsy has been performed in 63 cases of chronic lymphocytic leukaemia (CLL). Four different histological patterns were observed: (a) interstitial (lymphoid infiltration without displacement of fat cells) in 12 cases; (b) nodular (abnormal lymphoid nodules without interstitial infiltration) in 10 cases; (c) mixed (combination of the first two patterns) in 21 cases; and (d) diffuse (replacement of both haemopoietic and fat cells by lymphoid infiltration) in 20 cases.
Statistical analysis of actuarial curves showed a significant difference of survival probability according to the bone marrow infiltration patterns. Thus, in patients with interstitial or nodular patterns the life expectancy is significantly longer than in those with mixed or diffuse patterns. Furthermore, a significant degree of correlation between bone marrow infiltration patterns and different methods of clinical staging in CLL was apparent.
The different bone marrow infiltration patterns in CLL probably reflects variations in the amount of lymphoid accumulation during the natural course of this disease. Because of its prognostic significance, bone marrow biopsy should have a place in CLL evaluation and staging.     

Differential cytotoxicity of an ether lipid on lymphoma and bone marrow cells and its role in purging malignant lymphoid cells from remission bone marrow contaminated with tumor cells.

Four human clonogenic malignant lymphoid cell lines (CEM, Su-DHL-4, Li-A, and Raji) as well as normal human bone marrow stem cell progenitor cells were investigated for clonal in vitro growth before and after incubation with the ether lipid ET-18-OCH3 for various times (1, 4, and 18 h) and at increasing concentrations of the drug (25, 50, 75, and 100 micrograms/ml). The clonal growth of the malignant lymphoid cell lines was inversely correlated with concentrations and times of drug incubation. The antineoplastic effect of ET-18-OCH3 was further amplified by subsequent cryopreservation. In a situation of 4-h exposure to less than or equal to 50 micrograms/ml ET-18-OCH3 and subsequent cryopreservation, in which greater than 50% of the normal human bone marrow progenitor cells survived, 1-3 logs of the malignant lymphoblastoid cells were killed, indicating a potential value of this drug for bone marrow purging in lymphoid malignancy. In order to simulate the situation of autologous bone marrow transplantation (ABMT) in complete remission of the disease, we contaminated normal human bone marrow cells with malignant CEM or Su-DHL-4 lymphoid cells at a ratio of 100:1. Results show that 4 h of incubation with 75 micrograms/ml ET-18-OCH3 and subsequent cryopreservation can eliminate 2-3 logs of clonogenic cells of the malignant lymphoblastoid cell lines under conditions that allow recovery of greater than 50% of the normal human hematopoietic progenitors.     

Occult lymphoma cells prevalent in autologous marrow from non-Hodgkin's diffuse lymphoma

The most effective treatment for recurrent non-Hodgkin's lymphoma (NHL) appears to be a high-dose cytotoxic chemotherapy (HDC) followed by autologous bone marrow transplantation (ABMT). However, it has been suggested that the presence of occult lymphoma cells in harvested marrow may be responsible for a significant fraction of treatment failures after HDC/ABMT. The present study examined randomly accrued NHL patients, independent of their cytogenic grades, for the presence of cells bearing bcl-2/immunoglobulin heavy chain (IgH) gene rearrangements in lymph node (LN) biopsies and the bone marrow by polymerase chain reaction (PCR) and Southern blot hybridization combined with a classical culturing technique. Among 41 NHL patients examined, bcl-2/IgH translocations were evident in LN biopsies and marrow from each of 10 follicular lymphoma patients, but not in any samples from 31 newly diagnosed diffuse lymphoma patients. Marrow aspirates from several patients that were cultured using a one-week "triggering culture" followed by an extended period of conventional culture resulted in emergence of a monoclonal, IgH-rearranged, bcl-2-normal lymphoid cell population. Such outgrowth was specifically seen in cultures of diffuse lymphoma marrow (7 of 28 evaluable patients). Southern analysis for IgH rearrangement within LN biopsies and of cells cultured from marrow of individual diffuse lymphoma patients produced identical patterns, suggesting that the occult lymphoma cells present in harvested marrow were derived from the predominant lymphoma cell population represented within involved lymph nodes. The culture of histologically occult lymphoma from diagnostic marrow and analysis of the derived cells by Southern blot hybridization can be used to detect potentially aggressive lymphoma cells within harvested marrow, despite their lack of bcl-2 gene rearrangement.     

Cytogenetic Differences between Bone Marrow and Spleen in a Case of Agnogenic Myeloid Metaplasia Developing Blast Crisis

Chromosome studies of cells from bone marrow and spleen were performed in a patient with agnogenic myeloid metaplasia (MM) developing blast crisis after a chronic phase lasting for 6.5 years. The proportions of blast cells were roughly the same in bone marrow and spleen. In the bone marrow 43% of the metaphases were abnormal with a marker chromosome whereas all spleen metaphases were normal. The results indicate that chromosomal changes associated with blast transformation in MM may occur in the bone marrow prior to such changes in the spleen and support the concept that bone marrow and spleen may constitute relatively separate pools of haemopoietic tissue in chronic myeloproliferative diseases.     

Bone marrow histological changes in myelodysplastic syndrome and its clinical significance]

The authors compared bone marrow histological changes in 28 cases of myelodysplastic syndrome (MDS), 21 cases of aplastic anemia and 8 cases of hemolytic anemia. It is shown that abnormal localization of immature precursors (ALIP) is a characteristic change in MDS. The presence of erythroblastic islands and the variance of morphology of megakaryocytes are valuable for diagnosis. The histological method for the observation of lymphoid micromegakaryocytes is not so accurate as cytological method. "ALIP" can more or less help to evaluate the prognosis of MDS. According to the histological changes, it is easy to differentiate the three types of anemia we studied.     

A cell-autonomous requirement for CXCR4 in long-term lymphoid and myeloid reconstitution

Mice lacking the chemokine stromal cell-derived factor/pre-B cell growth stimulating factor or its primary physiological receptor CXCR4 revealed defects in B lymphopoiesis and bone marrow myelopoiesis during embryogenesis. We show here that adoptive transfer experiments reveal a deficiency in long-term lymphoid and myeloid repopulation in adult bone marrow by CXCR4-/- fetal liver cells, although stromal cell-derived factor/pre-B cell growth stimulating factor-/- fetal liver cells yield normal multilineage reconstitution. These findings indicate that CXCR4 is required cell autonomously for lymphoid and myeloid repopulation in bone marrow. In addition, CXCR4-/- fetal liver cells generated much more severely reduced numbers of B cells relative to other lineages in bone marrow. Furthermore, the repopulation of c-kit+ Sca-1(+) linlow/- cells by CXCR4-/- fetal liver cells was less affected compared with c-kit+ Sca-1(-) linlow/- cells. By previous studies, it has been shown that c-kit+ Sca-1(+) linlow/- cells are highly purified primitive hematopoietic progenitors and that c-kit+ Sca-1(-) linlow/- cells are more committed hematopoietic progenitors in mice. Thus, CXCR4 may play an essential role in generation and/or expansion of early hematopoietic progenitors within bone marrow.     

Bone marrow immunophenotyping by flow cytometry in refractory cytopenia of childhood

Refractory cytopenia of childhood is the most common type of childhood myelodysplastic syndrome. Because the majority of children with refractory cytopenia have a normal karyotype and a hypocellular bone marrow, differentiating refractory cytopenia from the immune-mediated bone marrow failure syndrome (very) severe aplastic anemia can be challenging. Flow cytometric immunophenotyping of bone marrow has been shown to be a valuable diagnostic tool in differentiating myelodysplastic syndrome from non-clonal cytopenias in adults. Here, we performed the first comprehensive flow cytometric analysis of immature myeloid, lymphoid cells and erythroid cells, and granulocytes, monocytes, and lymphoid cells in bone marrow obtained from a large prospective cohort of 81 children with refractory cytopenia. Children with refractory cyotopenia had a strongly reduced myeloid compartment, but not as severe as children with aplastic anemia. Furthermore, the number of flow cytometric abnormalities was significantly higher in children with refractory cytopenia than in healthy controls and in children with aplastic anemia, but lower than in advanced myelodysplastic syndrome. We conclude that flow cytometric immunophenotyping could be a relevant addition to histopathology in the diagnosis of refractory cytopenia of childhood. (The multi-center studies EWOG-MDS RC06 and EWOG-MDS 2006 are registered at clinicaltrials.gov identifiers 00499070 and 00662090, respectively).     

Bone marrow transplantation for the treatment of immune deficiency states

The first successful allogeneic bone marrow transplants were performed in children with severe combined immune deficiency (SCID). Bone marrow transplants for patients with SCID have been in the forefront of clinical bone marrow transplantation including the first successful use of T lymphocyte-depleted haploidentical bone marrow and matched unrelated donors. Successful bone marrow transplantation for most forms of SCID requires only the engraftment of donor lymphoid stem cells; donor hematopoietic stem cell engraftment is usually not required. The Wiskott-Aldrich syndrome was the first genetic disease involving the hematopoietic stem cell to be completely corrected by allogeneic bone marrow transplantation. The successful transplantation of Wiskott-Aldrich syndrome patients demonstrated that agents with adequate anti-lymphoid and hematopoietic stem cell activity were necessary in order to achieve complete donor lymphoid and hematopoietic stem cell engraftment. Initially, total body irradiation and now busulfan are used to ablate recipient hematopoietic stem cells, while cyclophosphamide is used to ablate recipient lymphoid stem cells. No single agent/drug is capable of eliminating both stem cell populations. Histocompatible bone marrow transplantation has a role in the treatment of patients with immune deficiency due to primary defects of the hematopoietic stem cell. The recent introduction of cytokines (gamma-interferon and granulocyte colony stimulating factor) may reduce the need for bone marrow transplantation for myeloid immune deficiency states. Initial attempts to treat patients with the acquired immune deficiency syndrome by bone marrow transplantation were limited by the lack of effective concomitant anti-viral therapy. Bone marrow transplantation for immune deficiency states continues to be in the forefront of human bone marrow transplantation.     

Hairy cell leukemia and other "hairy-cell" lymphoproliferative disorders (LPD-HC): the significance of morphologic, histologic and immunologic studies

In this report the clinical, morphologic, histologic and immunologic findings of 41 patients with hairy lymphoid cells in peripheral blood and/or bone marrow are analyzed. In 27 patients the diagnosis of hairy cell leukemia was established. 14 patients had other variants of lymphoproliferative disorders: malignant lymphoma with hairy cells--7, chronic lymphocytic leukemia with hairy cells--5, and T cell lymphoproliferative disorder with hairy cells--2 patients, respectively. Several variants of malignant lymphoma with hairy cells were defined: lymphocytic, centrocytic and lymphoplasmacytic. The importance of combined use of bone marrow biopsy and immunophenotyping for the correct diagnosis of hairy cell leukemia and other "hairy-cell" lymphoproliferative disorders is stressed. The obtained data suggest relationship between characteristic clinical manifestation (isolated splenomegaly), presence of hairy cells and CD11c-antigen expression.     

Susceptibility of human fetal mesenchymal stem cells to Kaposi sarcoma-associated herpesvirus

Recent reports link Kaposi sarcoma-associated herpesvirus (KSHV) infection of bone marrow cells to bone marrow failure and lymphoproliferative syndromes. The identity of the infected marrow cells, however, remains unclear. Other work has demonstrated that circulating mononuclear cells can harbor KSHV where its detection predicts the onset and severity of Kaposi sarcoma. In either setting, bone marrow precursors may serve as viral reservoirs. Since mesenchymal stem cells (MSCs) in human bone marrow regulate the differentiation and proliferation of adjacent hematopoietic precursors, we investigated their potential role in KSHV infection. Our results indicate that primary MSCs are susceptible to both cell-free and cell-associated KSHV in culture. Moreover, infection persisted within nearly half of the cells for up to 6 weeks. Thus, MSCs possess a clear capacity to support KSHV infection and warrant further exploration into their potential role in KSHV-related human disease.