The influence of age and breeding conditions on the number and proliferation of intraepithelial lymphocytes in pigs.

The number and subset composition of intraepithelial lymphocytes (IEL) were studied in pigs in different age groups ranging from 1 day to 4.5 years. There were no major differences between the numbers of IEL in the jejunum and ileum. The postnatal increase of IEL largely depended on the breeding conditions: in germ-free animals there was a constant level, while in conventionally bred pigs the numbers increased more obviously than in specified pathogen-free (SPF) pigs. As the numbers of IEL can rise due to increased influx from other organs, decreased emigration, decreased apoptosis or local proliferation, the incorporation of the DNA precursor bromodeoxyuridine (BrdU) by IEL was studied after different labelling protocols. There were low but definite numbers of BrdU+ IEL 1 h after BrdU, indicating in-situ proliferation in conventional animals with a tendency to a higher index in the jejunum than the ileum. Repetitive labelling protocols for 14 days resulted in 12-20% BrdU+ IEL, which may be caused by local proliferation and immigration of lymphocytes produced in other lymphoid organs as documented for the pig. Future studies need to focus on the factors regulating local proliferation and the migration of IEL in different species.