Inhibition of human monocyte function by prophylactic doses of chloroquine

Monocyte phagocytosis of malaria β-haematin in the presence of artemisinin, amodiaquine, chloroquine, doxycycline, primaquine, pyrimethamine and quinine

The intraerythrocytic malaria parasite digests haemoglobin to provide amino acids for metabolism and releases toxic haem that is sequestered into haemozoin, a non-toxic, insoluble, crystalline pigment. Following erythrocyte rupture, haemozoin is released into circulation and phagocytosed by monocytes. Phagocytosed haemozoin and antimalarial drugs have both been reported to modulate monocyte functions. This study determined the effects of therapeutic concentrations of seven antimalarial drugs; amodiaquine, artemisinin, chloroquine, doxycycline, primaquine, pyrimethamine and quinine, on the phagocytosis of β-haematin (synthetic haemozoin) by two monocytic cell lines, J774A.1 and U937, and human peripheral blood mononuclear cells. A novel spectrophotometric method based on the absorbance (O.D 400 nm) of alkali/SDS treated monocytes containing β-haematin was developed to complement counting phagocytosis with microscopy. The method has potential use for the large scale screening of monocyte phagocytic activity. Artemisinin, quinine, primaquine and pyrimethamine activated β-haematin phagocytosis by 12% or more, whereas amodiaquine, chloroquine and doxycyline inhibited β-haematin phagocytosis. In contrast, antimalarial drugs had minimal inhibitory effects on the phagocytosis of latex beads with only quinine resulting in more than 20% inhibition. Antimalarial drugs appear to alter monocyte phagocytic activity which has implications for the treatment, pathogenicity and adjunct therapies for malaria.

Plasmodium falciparum: effect of antimalarial drugs, malaria pigment (β-haematin) and Plasmodium falciparum lysate on monocyte GTP-cyclohydrolase 1 gene expression

In interferon-γ activated human macrophages, GTP-cyclohydrolase 1 catalyses the conversion of guanosine triphosphate to 7,8-dihydroneopterin triphosphate, which is dephosphorylated and oxidized to form neopterin. Elevated levels of neopterin have been detected in the urine and serum of malaria-infected patients. In this study, U937 cells were treated with interferon-γ and one of the following antimalarial drugs: amodiaquine, artemisinin, chloroquine, doxycycline, primaquine, pyrimethamine or quinine. The effects of treating the U937 cells with malaria pigment (β-haematin), latex beads, or Plasmodium falciparum-infected-red blood cell lysates were also investigated. U937 GTP-cyclohydrolase 1 mRNA expression was monitored using reverse-transcriptase-quantitative PCR. Artemisinin, primaquine, and quinine down-regulated GTP-cyclohydrolase 1 gene expression 1.26-, 1.29-, and 1.63-fold, respectively. The remaining drugs had insignificant effects. β-haematin up-regulated GTP-cyclohydrolase 1 mRNA expression 1.18-fold, whereas P. falciparum-infected red blood cell lysate down-regulated expression 1.56-fold. These results show the differing immunomodulatory actions of antimalarial drugs and malaria pigment taking place in monocytes.

Chloroquine enhances the number of IL-10 producing cells and the expression of B7-2 and ICAM-1 in in vitro-cultured PBMC

Chloroquine is prescribed as both an anti-malarial and an anti-inflammatory drug. However, its immuno-modulating effects remain largely unclear. Previous studies have shown that chloroquine inhibits antigen-induced proliferation, implying immuno-suppressive effects. In this study, we examined whether the inhibition of the proliferation reflects changes in the surface molecules that are important for T-cell activation and whether chloroquine affects the balance between pro- and anti-inflammatory cytokines. Chloroquine elevated the expression of the costimulatory and adhesion molecules B7-2 (CD86) and ICAM-1 (CD54) in peripheral mononuclear cells (PBMC). An increased percentage of CD14+ cells was also observed, and within this cell population, an increase in ICAM-1 expression was revealed by double-staining experiments. Assessment of the frequencies of interleukin (IL)-10 and interferon (IFN)-gamma-producing cells in in vitro-cultivated PBMCs showed that the ratio between pro- and anti-inflammatory cytokines changed after exposure to chloroquine, favouring anti-inflammatory immune responses. This effect was mainly because of increased frequencies of IL-10-producing cells and was seen with or without the presence of stimulating antigens or mitogens. Our findings indicate that chloroquine affects the direction of the lymphocyte stimulation towards an anti-inflammatory response by affecting the antigen-presenting cells (APC) and the balance between pro- and anti-inflammatory cytokines, rather than generally inhibiting cytokine production.

Antimalarial drugs modulate the expression of monocyte receptors

The cytoadherence of four Plasmodium falciparum malaria isolates (FCR-3, RSA-14, 15 and 17) to monocytes was used as a measure of the expression of monocyte receptors after the monocytes had been exposed to seven antimalarial drugs. Quinine, chloroquine, primaquine, pyrimethamine, artemesinin, mefloquine and proguanil all down-regulated the expression of monocyte receptors by 40% or greater at the therapeutic concentrations of each drug. Each malaria isolate had a unique adherence profile for drug induced changes in monocytes. Each drug appeared to alter the expression of more than one monocyte receptor. The most effective drugs were quinine, pyrimethamine and palludrin and the least effective were artemether and mefloquine. The results suggest a previously undetected immunomodulatory action of antimalarial drugs.

Monocyte activation in rheumatoid arthritis (RA): increased integrin, Fc gamma and complement receptor expression and the effect of glucocorticoids

The aim of this work was to study the expression of beta 1- and beta 2-integrins, CR1, CD44 and Fc gamma receptors on peripheral blood monocytes in RA. The expression of these receptors was measured by flow cytometry, before and after treatment with low-dose prednisolone. Expression of the same receptors was also measured before and after treatment with metyrapone, a substance that inhibits the synthesis of cortisol in the adrenals. The expression of the beta 2-integrins CD11a, CD11b and CD18, of CD35 (CR1), and of Fc gamma RII and Fc gamma RI (CD32 and CD64) on monocytes was elevated in the RA patients compared with healthy controls, while the expression of the beta 1-integrins (CD29, CD49d, CD49f) was unaffected. A significant correlation between monocyte expression of CD64 and C-reactive protein (CRP), and blood platelet count, respectively, was found in the group of patients with RA. After 4-6 weeks of treatment with low-dose prednisolone, the expression on the monocytes of CD11a, CD11b, CD18, CD35, CD32 and CD64 was normalized. A significant correlation (r = 0.64, P = 0.02) was found between the decrease in expression of CD11b and clinical improvement after prednisolone treatment. Two days of metyrapone treatment, which significantly lowered the serum cortisol levels, elevated the expression of CD35 and CD49f. Priming of peripheral monocytes seems to be one of the mechanisms behind the recruitment of monocytes to the rheumatoid synovium. One reason for the good clinical effects of prednisolone in RA could be a down-regulation of adhesion and phagocytosis receptors on monocytes.

Action of chloroquine on nitric oxide production and parasite killing by macrophages

Chloroquine is known to inhibit several functions of macrophages, but its effect on the nitric oxide (NO)-dependent parasite killing capacity of macrophages has not been documented. NO synthesis by interferon-gamma-induced mouse and casein-elicited rat macrophages was significantly and irreversibly inhibited by chloroquine. The activity of the inducible NO synthase was not directly altered, but previous incubation of macrophages with chloroquine decreased it. Chloroquine did not alter arginase activity or arginine uptake. NADPH diaphorase activity, an indicator of NO synthase was impaired. Western blotting showed that inducible NO synthase synthesis was blocked by chloroquine. The blocking of NO formation by chloroquine resulted in increased infection of mouse peritoneal macrophages by Trypanosoma cruzi (T. cruzi). This suggests that chloroquine decreases NO formation by macrophages by inhibiting the induction of NO synthase. The findings are further evidence that NO is involved in the anti-parasitic response of macrophages.

Malaria: drug use and the immune response

Malaria is a controllable disease, yet the resources required - human, technical and financial - are massive, and are currently beyond the vast majority of the 96 countries where the disease is endemic. The control measures most widely applied are vector control through spraying or use of insecticide-impregnated bednets, and chemotherapy. The biological problems to add to the resource issues are well known; increasing resistance of anopheline mosquitoes to the most widely used insecticides, and the progressive development of drug resistance in the parasite populations, especiallyPlasmodium falciparum.