Interleukin-12 as an adjuvant promotes immunoglobulin G and type 1 cytokine recall responses to major surface protein 2 of the ehrlichial pathogen Anaplasma marginale

Anaplasma marginale is a tick-transmitted pathogen of cattle closely relate d to the human ehrlichiae, Ehrlichia chaffeensis and the agent of human gra nulocytic ehrlichiosis (HGE). These pathogens have in common a structurally conserved outer membrane protein (OMP) designated the major surface protei n 2 (MSP-2) in A, marginale and HGE and OMP-1 in E. chaffeensis. Protective immunity against ehrlichial pathogens is believed to require induction of gamma interferon (IFN-gamma) and opsonizing immunoglobulin (Ig) subclasses directed against OMP epitopes that, in concert, activate macrophages for ph agocytosis and killing. Because interleukin-12 (IL-12) acts as an adjuvant for protein immunization to induce IFN-gamma and protective immunity agains t intracellular pathogens, we hypothesized that as an adjuvant with MSP-2, IL-12 would augment type 1 recall responses to A. marginale. IL-12 was coad sorbed with MSP-2 to alum and shown to significantly enhance IFN-gamma prod uction by lymph node cells (LNC) and LNC-derived CD4(+) T-cell lines from i mmunized calves following recall stimulation with A. marginale, LNC prolife ration and IL-2 production were also enhanced in IL-12-treated calves. Elev ated recall proliferative responses by peripheral blood mononuclear cells w ere still evident 9 months after immunization. Serum IgG levels were consis tently increased in IL-12 immunized calves, predominantly due to higher IgG 1 responses. The results support the use of IL-12 coadsorbed with OMP of eh rlichial pathogens in alum to amplify both antibody and type-1 cytokine res ponses important for protective immunity.