The voltage-gated Kv1.3 K ϩ channel is a novel target for immunomodulation of autoreactive effector memory T (T EM ) cells that play a major role in the pathogenesis of autoimmune diseases. We describe the characterization of the novel peptide ShK(L5) that contains L-phosphotyrosine linked via a nine-atom hydrophilic linker to the N terminus of the ShK peptide from the sea anemone Stichodactyla helianthus. ShK(L5) is a highly specific Kv1.3 blocker that exhibits 100-fold selectivity for Kv1.3 (K d ϭ 69 pM) over Kv1.1 and greater than 250-fold selectivity over all other channels tested. ShK(L5) suppresses the proliferation of human and rat T EM cells and inhibits interleukin-2 production at picomolar concentrations. Naive and central memory human T cells are initially 60-fold less sensitive than T EM cells to ShK(L5) and then become resistant to the peptide during activation by up-regulating the calcium-activated K Ca 3.1 channel. ShK(L5) does not exhibit in vitro cytotoxicity on mammalian cell lines and is negative in the Ames test. It is stable in plasma and when administered once daily by subcutaneous injection (10 g/kg) attains "steady state" blood levels of ϳ300 pM. This regimen does not cause cardiac toxicity assessed by continuous EKG monitoring and does not alter clinical chemistry and hematological parameters after 2-week therapy. ShK(L5) prevents and treats experimental autoimmune encephalomyelitis and suppresses delayed type hypersensitivity in rats. ShK(L5) might prove useful for therapy of autoimmune disorders.

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Bass hepcidin was purified from the gill of hybrid striped bass (Morone chrysops ؋ Morone saxatilis) based on antimicrobial activity against Escherichia coli. This 21-amino acid peptide has 8 cysteines engaged in 4 disulfide bonds and is very similar to human hepcidin, an antimicrobial peptide with iron regulatory properties. To gain insight into potential role(s) of bass hepcidin in innate immunity in fish, we synthesized the peptide, characterized its antimicrobial activities in vitro, determined its solution structure by NMR, and quantified hepatic gene expression in vivo following infection of bass with the fish pathogens, Streptococcus iniae or Aeromonas salmonicida. Its structure is very similar to that of human hepcidin, including the presence of an antiparallel ␤-sheet, a conserved disulfide-bonding pattern, and a rare vicinal disulfide bond. Synthetic bass hepcidin was active in vitro against Gram-negative pathogens and fungi but showed no activity against key Gram-positive pathogens and a single yeast strain tested. Hepcidin was non-hemolytic at microbicidal concentrations and had lower specific activity than moronecidin, a broad spectrum, amphipathic, ␣-helical, antimicrobial peptide constitutively expressed in bass gill tissue. Good synergism between the bacterial killing activities of hepcidin and moronecidin was observed in vitro. Hepcidin gene expression in bass liver increased significantly within hours of infection with Gram-positive (S. iniae) or Gram-negative (A. salmonicida) pathogens and was 4 -5 orders of magnitude above base-line 24 -48 h post-infection. Our results suggest that hepcidin plays a key role in the antimicrobial defenses of bass and that its functions are potentially conserved between fish and human.