During the early neonatal life, important changes occur in the gut. Challenges from milk and the microbial flora conduct mucosal immune system maturation. Lamina propria lymphocytes (LPL) and intraepithelial lymphocytes (IEL) constitute the non-aggregated gut-associated lymphoid tissue (GALT) acting as effector cells. Although it has been described that LPL are phenotypically and functionally similar to peripheral lymphocytes, little is known about the ontogenesis of this GALT population in the rat.

The aim of the present study was to establish the phenotypic maturation of LPL in suckling Lewis rats, from day 1 up to day 21 after birth. The study was in compliance with the guidelines for the care and use of laboratory animals approved by the Ethic Commission for Animal Experimentation of the University of Barcelona. Rats were killed by decapitation. LPL were isolated from the small intestine by previously removing IEL and subsequent enzymatic digestion. Phenotype was assessed by FACS analysis, using double labelling techniques and monoclonal antibodies for detection of lymphocytes (CD45+), B cells (CD45RABC+, IgA+ or κ chain+), T subsets (CD4+, CD8+, CD5+, CD3+, TCRαβ+), and NK cells (NKR-P1+). The surface expression of activation and adhesion molecules (CD25, CD90 and CD2) was also determined.

CD45+ LPL (total lymphocytes) showed a progressive increase in percentage with the age of animals being more than 80 % of gated lymphocytes from day 14. In this CD45+ population, the highest percentage corresponded to CD8+ LPL (50-60 %) during the first two weeks. B cell proportion increased up to more than 35 % just before weaning. CD4+ LPL percentage showed a positive evolution during the studied period. Most of CD4+ LPL cells were CD5+, CD3+, CD2+ and TCRαβ+, although proportions varied along suckling period. About 20-30 % of CD4+ LPL were also positive for CD90. In addition, there were 20 % of CD4 LPL that also expressed CD25. CD8+ LPL cells include lymphocytes bearing CD8 molecule as the homodimer αα or the heterodimer αβ. Most of the CD8 T cells were CD8 αα until day 11, but later the ratio αα/αβ tended to be 1. Moreover, most of CD8+ LPL were CD5-, TCRαβ+ and CD3+ although proportions varied along the suckling period. NK cells represented about 20 % of LPL on day 14, and decreased thereafter. Only a half of NK LPL shared CD8 positivity. In summary, this study shows that the LPL phenotype does effectively change during the suckling period when the immune system is still maturing.

This work was supported by the Ministerio de Educación y Cultura (AGL-2000-0913).