systemic reaction characterized by fever, leukocytosis, increase in erythrocyte sedimentation rate, increases in Leucocytosis secretion of ACTH and glucocorticoids, activation of Complement activat complement and clotting cascades, decreases in serum levels of iron and zinc, a negative nitrogen balance, and by dramatic changes in the concentration of some plasma ,l' proteins. These proteins are named acute phase proteins. i

Interleukin-6 and the acute phase response

Interleukin 6 (IL-6), promptly and transiently produced in response to infections and tissue injuries, contributes to host defense through the stimulation of acute phase responses, hematopoiesis, and immune reactions. Although its expression is strictly controlled by transcriptional and posttranscriptional mechanisms, dysregulated continual synthesis of IL-6 plays a pathological effect on chronic inflammation and autoimmunity. For this reason, tocilizumab, a humanized anti-IL-6 receptor antibody was developed. Various clinical trials have since shown the exceptional efficacy of tocilizumab, which resulted in its approval for the treatment of rheumatoid arthritis and juvenile idiopathic arthritis. Moreover, tocilizumab is expected to be effective for other intractable immune-mediated diseases. In this context, the mechanism for the continual synthesis of IL-6 needs to be elucidated to facilitate the development of more specific therapeutic approaches and analysis of the pathogenesis of specific diseases.

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IL-6 in Inflammation, Immunity, and Disease

Publisher Summary This chapter focuses on structure and expression of interleukin-6 (IL-6), which is a cytokine with pleiotropic activities that plays a central role in host defense. IL-6 can exert growth-inducing, growth-inhibitory, and differentiation-inducing activities, depending on the target cells. These activities include (1) terminal differentiation (secretion of immunoglobulins) in B cells and (2) growth promotion on various B cells. IL-6 has been implicated in the pathology of many diseases including multiple myeloma, mesangial proliferative glomerulonephritis, rheumatoid arthritis, and acquired immunodeficiency syndrome (AIDS). Selective inhibition of the synthesis or of the action of IL-6 may have therapeutic benefit against the IL-6-associated diseases. On the other hand, IL-6 has potent antitumor activity against certain types of tumors. Application of IL-6 is promising in cancer treatment as well as in treatment of radiation- or chemotherapy-induced myelosuppression. The cell biology of the intracellular events that link transduction to gene regulation is an important area, and work on these topics helps to understand such phenomena as multifunction of IL-6 and bidirectional effects of cell growth depending on the cell type.

Interleukin-6 in biology and medicine

Molecular cloning and expression of an IL-6 signal transducer, gp130

The major regulator of circulating platelet levels is believed to be a cytokine termed thrombopoietin. It is thought to be a lineage-specific cytokine affecting the proliferation and maturation of committed cells resulting in the production of megakaryocytes and platelets. Despite considerable efforts by a number of laboratories, the unequivocal identification of thrombopoietin has proven elusive. Here we report the functional cloning of a murine complementary DNA encoding a ligand for the receptor encoded by the c-mpl proto-oncogene (c-Mpl). The encoded polypeptide has a predicted molecular mass of 35,000 (M(r) 35K). The protein has a novel two-domain structure with an amino-terminal domain homologous with erythropoietin and a carboxy-terminal domain rich in serine, threonine and proline residues and containing seven potential N-linked glycosylation sites. Intraperitoneal injections of mice with recombinant protein increase circulating platelet levels by greater than fourfold after 7 days. These results along with those presented in the accompanying report strongly suggest that the ligand for c-Mpl is thrombopoietin.

Cloning and expression of murine thrombopoietin cDNA and stimulation of platelet production in vivo

The response of cells of the murine megakaryocytic lineage to human interleukin 6 (IL-6) was assessed in serum-depleted cultures using a variety of biological assays. IL-6 alone had no influence on megakaryocytic colony formation but augmented the numbers of these colonies induced by the multipotent colony-stimulating factor interleukin 3. However, in liquid marrow cultures, IL-6 alone promoted marked increments in megakaryocytic size and the activity of acetyl-cholinesterase, a marker enzyme of the lineage. Moreover, IL-6 induced a significant shift toward higher ploidy classes when megakaryocytic DNA was quantitated by flow cytometry. To determine whether the influence of IL-6 on megakaryocytic maturation was direct, the factor was added to cultures of single megakaryocytes isolated from megakaryocytic colonies. Fifty-four percent of these cells increased in size compared with 19% of those grown without IL-6. The data show that human IL-6 is a potent direct-acting growth factor for murine megakaryocytes with activity promoting maturation of that lineage.

https://www.pnas.org/content/pnas/86/15/5953.full.pdf

Human interleukin 6 is a direct promoter of maturation of megakaryocytes in vitro.

Interleukin-6 is a potent thrombopoietic factor in vivo in mice.

The response of cells of the megakaryocytic lineage to interleukin 6 (IL6), a cytokine with multiple biological activities, was studied by adding the factor to human bone marrow (BM) cultures. IL6 alone had no effect on megakaryocytic colony formation in methylcellulose; however, in the presence of maximally stimulating concentrations of IL 3, almost a twofold increment in colony formation was observed. Tritiated thymidine suicide studies of BM incubated for 2 h with growth factors showed that almost one-half of megakaryocytic progenitors (CFU-Mk) preincubated with IL3 or IL3 plus IL6 were in S phase, whereas BM incubated with IL6 alone was similar to control (approximately 24% of CFU-Mk in S phase). When greater than or equal to 1 ng/ml of IL6 was added to liquid suspension cultures of BM, the size of individual megakaryocytes was significantly augmented compared with that seen in control cultures. Moreover, the DNA content of megakaryocytes grown in the presence of IL6 was increased (modal ploidy 16N) compared with cultures grown with IL3 (modal ploidy 8N). To determine if the effect of IL6 could be direct rather than mediated via accessory BM cells, the factor was added to cultures of isolated single megakaryocytes. Seventy-one percent of cells increased in diameter in the presence of IL6, while only 27% increased in size in the absence of the factor. The data show that IL6 is a direct maturation factor for human megakaryocytes, promoting increments in size and ploidy of these cells.

Interleukin 6 is a differentiation factor for human megakaryocytes in vitro

Interleukin-1 beta (IL-1 beta) induces thrombocytosis in mice: possible implication of IL-6

For the purpose of examining the mechanisms for the overproduction of megakaryocytes and platelets in essential thrombocythemia (ET), marrow megakaryocytic colony-forming units (CFU-Meg) were characterized using a methylcellulose culture system in nine patients with ET, and they were compared with those of 16 control subjects and four patients with secondary thrombocytosis (ST). The number of CFU-Meg per 10(5) cells in ET was significantly higher (p less than .001) than that in the controls (5.8 times of the controls) or that in ST. ET showed endogenous CFU-Meg (58 +/- 16% of the total CFU-Meg) that could form megakaryocytic colonies without phytohemagglutinin-stimulated leukocyte-conditioned medium (PHA-LCM) as a source of Meg-CSA. The controls and the patients with St revealed no endogenous CFU-Meg. The dose-response studies with PHA-LCM revealed that CFU-Meg from ET are highly sensitive to low concentrations of PHA-LCM compared to CFU-Meg from the control and ST. Erythropoietic burst-forming units (BFU-E) were monitored as a control progenitor, because the patients with ET studied did not have erythrocytosis. Although five out of nine patients with ET had endogenous BFU-E, the percentages of endogenous BFU-E were small (4-10%). The number of BFU-E in ET was not different from the control value. Thus, ET is associated with an enlarged CFU-Meg compartment that shows abnormal growth patterns when cultured in vitro, while the growth abnormality of BFU-E is small, which may underlie the hematological features of ET. The culture of CFU-Meg should be clinically useful for differentiating ET from ST.

Tatsumi Uchida

Papers

Human interleukin 6 is a direct promoter of maturation of megakaryocytes in vitro.

Interleukin-6 is a potent thrombopoietic factor in vivo in mice.

Interleukin 6 is a differentiation factor for human megakaryocytes in vitro

Interleukin-1 beta (IL-1 beta) induces thrombocytosis in mice: possible implication of IL-6

Megakaryocytic colony formation (CFU‐Meg) in essential thrombocythemia: Quantitative and qualitative abnormalities of bone marrow CFU‐Meg