Opsonisation and neutrophil phagocytosis in foals and adult horsesGröndahl, Gittan
Infectious diseases are the main cause of disease and deaths in foals. The neutrophil phagocyte is a part of the early, non-specific immune response, which is essential in defending the host against microbial invasion. Coating of microbes with opsonic factors such as antibodies and complement from serum is required for optimal phagocytosis.
Flow cytometric methods were developed and used for studies of equine neutrophil phagocytosis and serum opsonisation of Saccharomyces cerevisiae, Escherichia coli and Actinobacillus equuli, and of the expression of complement receptor subunit CD18 on neutrophils. Foals from birth to 2 months of age and adult horses were studied.
Incubation for 15 min was found sufficient for both opsonisation and phagocytosis. Opsonisation of yeast and A. equuli, but not of E. coli, was observed already at low serum concentrations (<3%).
Complement activation was needed for optimal phagocytosis of all the microbes studied. The classical pathway was required for yeast phagocytosis at low serum concentrations (1.5%) and is the main pathway for C3 deposition with <50% serum. At higher serum concentrations, the alternative pathway is able alone to provide sufficient amounts of C3. The main form of C3 on yeast cells is iC3b and the rest is C3b. The molecular weights of equine C3 fragments were similar to those of their human equivalents.
In newborn foals, colostrum ingestion was required to achieve a serum opsonic capacity for all the microbes studied. Serum from foals up to 3-4 weeks of age showed a lower capacity to opsonise yeast, but a higher capacity to opsonise E. coli compared to serum from older foals and horses. No age-related differences were observed in serum opsonisation ofA. equuli. Very low serum concentrations of IgG or IgGb were associated with decreased phagocytosis of yeast and E. coli, but not of A. equuli. However, there were large individual variations in the opsonisation of yeast, irrespective of the concentrations of IgG, by sera from newborn foals and plasma from individual adult donors. This may have been due to different levels of complement activation. Plasma transfusion to healthy 7-day old foals resulted in an increased serum capacity for yeast opsonisation at 14 days.
The neutrophil phagocytosis capacity per se was similar in foals and adult horses, and the expression of complementreceptor CD18 was also similar, or higher in the foals.
These results emphasise the importance of both antibodies and complement factors in serum for an effective defence against microorganisms in young foals. Intravenous administration of plasma to foals seems to be of some benefit, but additional studies are warranted to characterise the factors involved. Important differences were observed between various microbes in the particular requirements for effective phagocytosis.
The involvement of complement in common equine pathological conditions deserves further attention. A better understanding of the equine complement system may lead to improved prophylactic and therapeutic regimens in foals and adult horses.
Keywordsneutrophil; phagocytosis; opsonisation; foals; complement; IgG; transfusion; Escherichia coli; Actinobacillus equuli; colostrum; flow cytometry
Published inActa Universitatis Agriculturae Sueciae. Veterinaria
2001, number: 105
Publisher: Swedish University of Agricultural Sciences
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