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The ratio of full length IgY to truncated IgY in immune complexes affects macrophage phagocytosis and the acute phase response of mallard ducks (Anas platyrhynchos)

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Abstract

Ducks produce a full length IgY and a truncated isoform (IgYΔFc). IgY and IgY(ΔFc) were isolated from ducks vaccinated against Escherichia coli and used to make E.coli-Ig immune complexes (IC). Phagocytosis of IC by duck monocytes decreased directly with the proportion of IgYΔFc (p<0.001). IC containing IgY:IgY(ΔFc) at ratios of 100:0, 50:50, 0:100, and 0:0 (E. coli alone) were injected intravenously into naı̈ve mallard ducks. At 24 h after injection, plasma hemopexin levels were higher in ducks given either the 0:100 ratio or the 100:0 ratio than those given 50:50 or E. coli alone (p<0.05). Liver IL-1β mRNA levels followed a similar pattern. Splenic IL-1β mRNA decreased markedly as the proportion of IgY(ΔFc) increased (p<0.01). Thus, IgY(ΔFc) may shift the response to IC from the spleen to the liver as infections progress from acute to chronic.

Introduction

Ducks have an antibody response that differs markedly from that of chickens. Ducks produce two isoforms of the immunoglobulin (Ig) ν-chain, a full length isoform (IgY), and a truncated isoform (IgYΔFc) [1], [2], while chickens produce only the full length form. Both IgY and IgY(ΔFc) are produced from a single gene by alternative mRNA processing pathways, and IgY(ΔFc) lacks the two C-terminal domains (Cν3, Cν4) of the heavy chain [3]. Genomic organization of the duck Ig heavy chains has been characterized [4], [5] but the mechanism that regulates the production of IgY or IgY(ΔFc) has not been determined. Most secondary effector functions of Ig are mediated by the Fc region. Thus, IgY(ΔFc) is incapable of opsonization and complement (C′) fixation [6], [7] and presumably is primarily involved in neutralization.

Efficient Ig-mediated elimination of antigen occurs through the formation of immune complexes (IC) consisting of antigen, Ig, and C′ proteins when the C′-activating Ig is present [8]. The liver and spleen provide the greatest contribution to IC clearance through the interaction of IC with resident mononuclear phagocytic cells [9], [10], [11]. IC are cleared either by Fc or C′-mediated pathways, which elicit differing types of responses [12], [13]. In mammals, Fc-mediated phagocytosis of IC by mononuclear phagocytes causes a pro-inflammatory response by stimulating the production and secretion of interleukin-1β (IL-1β), tumor–necrosis factor-α (TNF-α), and interleukin-6 (IL-6) [12], while C′-mediated phagocytosis fails to stimulate a pro-inflammatory response [13]. Therefore, the mechanism of IC clearance can affect the type of immune response elicited.

The pro-inflammatory cytokines IL-1β, TNF-α, and IL-6 cause metabolic and behavioral alterations referred to as the acute phase response and include fever, anorexia, lethargy, and somnolence [13], [14], [15], [16]. The acute phase response bolsters immunity by recruiting immune cells and hepatic secretion of acute phase proteins that aid in host defense [17], [18]. There is a metabolic cost to the acute phase response and its down-regulation is associated with faster growth in young chicks [15], [19].

While previous research has characterized the duck Ig response [1], [20], [21], [22] and inability of IgY(ΔFc) to perform secondary effector functions [6], the relative ability of IgY and IgY(ΔFc) to activate the acute phase response has not been examined. The lack of the Fc region in duck IgY(ΔFc) would be predicted to weaken its ability to be processed by phagocytic cells and stimulate an acute phase response. Therefore, the effect of IC containing differing ratios of IgY:IgY(ΔFc) on duck peripheral blood monocyte (PBM) phagocytosis and stimulation of the acute phase response was examined.

Section snippets

Fixed Escherichia coli

E.coli (wild type K12; American Type Culture Collection, Manassas, VA) were grown in broth (8 g tryptone, 0.5 g NaCl, 1 l H2O) in an aerobic environment at 37 °C, 2.5 rpm. At 24 h the E. coli were washed three times (5000×g for 10 min) with saline and the pellet was resuspended in sterile saline with 10% neutral buffered formalin. The E. coli formalin suspension was mixed for 1 h, and then washed (7000×g for 10 min) four times with sterile saline.

Vaccination

One year old Mallard ducks (Anas platyrhynchos)

Yield and purity of anti-E. coli Ig

The total anti-E. coli (IgM plus both isoforms of IgY) in serum from ducks given a single injection of E. coli was 62 μg/ml and in serum from ducks given six booster vaccinations was 623 μg/ml. Following purification on Sephacryl, IgYΔFc had minor contamination with full length IgY when examined by SDS-gel electrophoresis. Purified anti-E. coli IgY had no detectable IgM or IgY(ΔFc).

Experiment 1

Phagocytosis decreased directly with the proportion of IgYΔFc (p<0.001), which can be described by the quadratic

Discussion

The production of both a full length IgY and a truncated isoform occurs in ducks and in a variety of other species including turtles and tortoises [25], [26], [27], [28], bullfrogs (Rana catesbiana) [29] and marine toads (Bufo marinus) [30]. Similar to duck IgY(ΔFc), turtle IgY(ΔFc) is unable to fix C′ or sensitize tissues for anaphylaxis [28]. Despite the recurrence of this seemingly ineffective molecule in a wide range of taxa, its functional value remains elusive.

Naı̈ve ducks produce

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