Cyclosporin as a probe for different modes of lymphocyte activation

Involution of bursa cloacalis (Fabricii) and thymus in Cyclosporin A-treated chickens

Fifty-day-old Arbor Acres chickens received a daily oral dose of 30 mg/kg Cyclosporin A for 21 consecutive days. This treatment induced the striking morphological changes in the bursa cloacalis (Fabricii). The number of bursal follicles did not change, but they were markedly decreased in size. The pars lymphoreticularis was collapsed and contained smaller accumulations of lymphocytes. The pars lymphoepithelialis was almost totally devoid of lymphocytes and it was composed of an empty network of epithelial cells. Interfollicular epithelium, follicle-associated epithelium, basement-membrane-associated epithelium and bursal epithelial cytoreticulum were not changed. The thymus also displayed substantial structural changes, whereby the thymic medulla was strongly reduced in size.

Cyclosporin A inhibits early mRNA expression of G0/G1 switch gene 2 (G0S2) in cultured human blood mononuclear cells

Cyclosporin A (CsA) may achieve its immunosuppressive effects by inhibiting the calcium- and calmodulin-dependent phosphatase calcineurin which is required for activation of target genes by members of the NFAT (nuclear factor of activated T cells) transcription factor family. Among these target genes is the gene encoding interleukin-2 (IL2), a cytokine facilitating progression through the G1 phase of the cell cycle. However, IL2 does not reverse CsA inhibition, suggesting that at least one other NFAT-sensitive gene may be involved. The human G0/G1 switch gene, G0S2, has potential NFAT-binding sites in the 5' flank and encodes a small basic potential phosphoprotein of unknown function. Using a sensitive, reverse transcription-polymerase chain reaction (RT-PCR) assay, G0S2 mRNA levels were assayed in cultured blood mononuclear cells. Freshly isolated cells contain high levels of G0S2 mRNA which rapidly decline. This "spontaneous stimulation" is also noted with some other G0S genes and has been attributed to some aspect of the isolation procedure. In cells that have been preincubated to lower mRNA levels, there is a transient increase in G0S2 mRNA, peaking between 1-2 h, in response to Concanavalin-A (ConA), or to the combination of phorbol ester (TPA), and the calcium ionophore, ionomycin. Both these responses are inhibited by CsA. Our results suggest that G0S2 expression is required to commit cells to enter the G1 phase of the cell cycle, and that, while not excluding other possible targets, early inhibition of G0S2 expression by CsA may be important in achieving immunosuppression. G0S2 may be of value as a reporter gene for analyzing the mechanism of action of CsA and its influence on the positive and negative selection of lymphocytes in response to self and not-self antigens.

Effect of cyclosporin A on the membrane potential and Ca2+ level of human lymphoid cell lines and mouse thymocytes

The effect of the immunosuppressive cyclosporin A (CsA) on the cytosolic free Ca2+ concentration ([Ca2+]i) and membrane potential of human B and T lymphoblastoid cells and mouse thymocytes was studied in order to reveal some features of the early stage of drug-cell interaction. Cytosolic free Ca2+ concentration of the cells was measured by spectrofluorimetry using indo-1 and quin2 fluorescent calcium indicators. Membrane potential was monitored in a flow cytometer with oxonol dye. CsA applied at 2-20 micrograms/ml final concentrations caused a dose-dependent, rapid, transient rise of [Ca2+]i in all cell types. This effect could be blocked by chelating the extracellular Ca2+ with EGTA but was not sensitive to Ca2+ channel blockers verapamil and nifedipine or K+ channel blocker 4-aminopyridine. A possible explanation for the calcium mobilizing effect of CsA is an ionophore-like mode of action at the cell membrane level. Besides directly interfering with mitogenic signals, the elevation of [Ca2+]i could be responsible for an initial hyperpolarization observed in CsA-treated T lymphocytes. This hyperpolarization, however, was not detectable in B lymphoblastoid cells. A further difference between B and T cells was the diverse pattern of depolarization following CsA treatment. This variance in the behaviour of T and B lymphocytes and the diversity of membrane transport systems in its background could account for the different final outcome of the drug-cell interaction.

T lymphocyte activation through the C28 pathway is insensitive to inhibition by the immunosuppressive drug FK-506

Nanomolar concentrations of the novel immunosuppressive drug FK-506 inhibit the proliferation of human T lymphocytes in vitro induced by mitogenic lectins or by monoclonal antibodies directed against the CD3 or CD2 surface antigens. However, the alternative pathway of T lymphocyte proliferation induced by monoclonal antibodies specific for CD28 together with phorbol esters is unaffected by FK-506.

The mechanism of action of the immunosuppressive drug FK-506

The novel immunosuppressive drug FK-506 inhibits the induction of lymphocyte proliferation in vitro by mitogenic combinations of phorbol esters and calcium ionophores. Early events inducible by phorbol esters alone are unaffected, while changes induced by calcium ionophores alone are completely suppressed by as little as 0.1 nM FK-506. The event in lymphocyte activation inhibited by FK-506 is completed early in the response. Its completion requires the provision of a Ca2+ signal and concurrent activation of protein kinase C and is accelerated as a function of the strength of protein kinase C activation.

Effects of cyclosporin A on humoral immunity to ticks and on cutaneous immediate and delayed hypersensitivity reactions to Ixodes ricinus L. salivary-gland antigens in re-infested rabbits

When rabbits are repeatedly infested with Ixodes ricinus L. adults, they acquire resistance to these ticks. A humoral response occurs and the skin of the host becomes progressively sensitized to the saliva of the ectoparasites. The present study examined the effects of cyclosporin A on these two aspects of immunity in re-infested rabbits. Using the enzyme-linked immunosorbent assay (ELISA), moderate suppression of the secondary IgG response to bites of these ticks was observed in animals given this immunosuppressive drug. The agent blocked the immediate cutaneous (type I) reaction normally developed in response to the intradermal injection of I. ricinus salivary gland antigens, as detected by measuring the skinfold thickness. The same method revealed a decreased delayed (type IV) hypersensitivity to these antigens. The present experiments demonstrate the major role of cyclosporin A-sensitive cells (mostly T-lymphocytes, mast cells and basophils) involved in the complex phenomenon of resistance to ticks.