Comparison of accessory cell functions of endothelial cells and monocytes: IL-2 production by T cells and PFC generation

Phenotypic overlap between monocytes and vascular endothelial cells

During embryonic development, endothelial cells (ECs) develop organ specific properties. ECs express specific markers, which are helpful in identifying these cells in vivo and in culture. Interestingly, most of the supposed specific endothelial markers are present on both ECs and hematopoietic precursors or mature blood cells, which correspond to the idea of a common embryonic precursor. Monocytes/makrophages and monocyte-derived dendritic cells, as more differentiated hematopoietic cell populations, show a wide phenotypic overlap with particularly hepatic sinusoidal, and microvascular endothelial cells within inflamed tissue, such as neovascularizised complicated atherosclerotic plaques. Furthermore, under local angiogenic growth conditions monocytes or monocyte precursors or immature dendritic cells may differentiate into endothelial like cells. First evidence suggests an endothelium-independent revascularization potential carried by monocyte-derived macrophages. These macrophages have been shown to form tunnel-like structures in ischemic regions. Future studies have to address the question, whether monocyte-/dendritic cell-derived endothelial like cells can develop a similar functional behaviour in vasoregulation, coagulation and fibrinolysis, as described for vascular endothelial cells, and thus may contribute to neoangiogenesis by a direct vessel-forming role.

Phenotypic overlap between hematopoietic cells with suggested angioblastic potential and vascular endothelial cells

The existence of angioblast-like circulating endothelial progenitor cells (EPC) in adult humans has been suggested recently. Their role in postnatal angiogenesis is under intensive investigation. Discrimination between the supposed angioblasts (AC133(+)/FLK-1(+)/CD34(+)) and mature endothelial cells (ECs) is complicated by the fact that subsets of hematopoietic cells express markers similar to those of ECs. Among these, monocytes/macrophages and monocyte-derived dendritic cells (DCs) are more differentiated hematopoietic cell populations. They show a wide phenotypic overlap with particularly sinusoidal and microvascular ECs. Furthermore, under local angiogenic growth conditions, monocytes or monocyte precursors or immature DCs may differentiate into endothelial-like cells (ELC). Initial evidence suggests an endothelium-independent revascularization potential carried by macrophages. These macrophages have been shown to form "tunnel-like structures" in ischemic regions. Future studies will need to address the question of whether monocyte-/dendritic cell-derived ELC can develop a similar functional behavior in vasoregulation, coagulation, and fibrinolysis, as described for vascular ECs, and thus may contribute to neoangiogenesis by a direct vessel-forming role.

Intravenous immunoglobulin in the treatment of paediatric cerebral malaria

Hyperimmune globulin can inhibit and reverse the cytoadherence between Plasmodium falciparum-infected erythrocytes and melanoma cells in vitro. Cytoadherence is believed to mediate disease in cerebral malaria. Therefore we studied the efficacy of i.v. immunoglobulin, purified from the plasma of local semi-immune blood donors, as an adjunct to standard treatment for cerebral malaria in Malawian children. The immunoglobulin preparation (IFAT antimalarial antibody titre 1:5120) recognized erythrocyte-associated antigens of each of 22 Malawian P. falciparum isolates studied, and reversed binding of Malawian isolates to melanoma cells. Immunoglobulin did not reverse binding to human monocytes or to cells of the human histiocytic lymphoma cell line U937. Thirty-one children with P. falciparum parasitaemia and unrousable coma were enrolled. All were treated with i.v. quinine dihydrochloride; in addition patients were randomized to receive either immunoglobulin (400 mg/kg by i.v. infusion over 3 h) or placebo (albumen and sucrose by similar infusion) in a double blind trial with sequential analysis. Of 16 patients receiving immunoglobulin, five (31%) died and five survivors had neurological sequelae. Of 15 patients receiving placebo, one (7%) died and two had sequelae. Parasite clearance, fever clearance and coma resolution times in survivors were similar in the two groups. Although the difference in outcome between the two groups was not significant, the trial was stopped because immunoglobulin was demonstrated not to be superior to placebo.

Accessory function of endothelial cells in anti-CD3-induced T-cell proliferation: synergism with monocytes

Monoclonal antibodies to CD3 can induce proliferation of resting T cells. In vitro this effect is dependent on the presence of monocytes. They serve as accessory cells providing a co-stimulatory signal after cross-linking of the antibody-coated TcR/CD3 complex by the Fc receptor on the monocytes. We have studied whether endothelial cells can replace monocytes with regard to this function. Highly purified T-cell preparations were cultured in the presence of anti-CD3 antibody, purified monocytes, and human umbilical vein endothelial cells. Anti-CD3 and endothelial cells alone were unable to support T-cell proliferation, due to lack of FcR expression. Addition, however, of as few as 1000 FcR+ monocytes (0.8% of the number of T cells present) to a coculture of T cells and endothelial cells (EC) in the presence of soluble anti-CD3 resulted in a strong proliferation of T cells. When anti-CD3 was presented in an immobilized form (coated to the culture well or to Sepharose beads), or when phytohaemagglutinin was added to the culture as a cross-linking agent, EC could support T-cell proliferation in the absence of any monocytes. We conclude that EC by themselves cannot support the proliferation of pure T cells induced by soluble anti-CD3, but are potent generators of the co-stimulatory signal(s). They provide a suitable starting material to further define this co-stimulatory activity.

HLA-DR expression in macaque neuroendothelial cells in vitro and during SIV encephalitis

HLA-DR expression in neuroendothelial cells (NEC) was studied during the course of SIV encephalitis in rhesus monkeys. HLA-DR determinants were detected on NEC in monkeys with SIV encephalitis, but not in control animals. In situ hybridization with an SIV probe indicated that HLA-DR expression was not a consequence of SIV replication within NEC. Cultured rhesus NEC stimulated with gamma interferon expressed HLA-DR to a higher degree than cultured brain fibroblasts or astrocytes. These data support the contention that NEC participate in retrovirus-induced inflammation and autoimmunity within the central nervous system.

Enhanced T-cell-activation due to combined stimulation by both endothelial cells and monocytes

Mitogen-induced interleukin 2 (IL-2) secretion by T cells was greatly enhanced by the combined presence of endothelial cells (EC) and monocytes (AC). EC also altered the time course of IL-2 secretion by peripheral blood mononuclear cells (PBM) in response to mitogens. The response to EC did not require expression of HLA-DR antigen and occurred equally well in response to autologous EC. At least one live accessory cell was required to induce T cells to secrete IL-2. When added to T cells plus live AC, both fixed EC and EC supernatant partially reproduced the enhancing effect of EC. Surface membrane IL-1 was present on both live and fixed EC. However, active EC supernatant contained no detectable IL-1 activity. Anti-IL-1 abolished the enhancement of T-cell responses by fixed but not live EC.