Early, microbially driven follicular reactions in the neonatal piglet do not contribute to expansion of the immunoglobulin heavy chain V-D-J repertoire.

Selective microbial colonisation of germ-free piglets is reported to result in expansion of immunoglobulin V(H)- and D(H)-segment usage from an initially limited repertoire. Here, the response of the palatine tonsil to microbial colonisation was compared in age-matched conventionally reared and germ-free piglets. At 3 and 5 days after birth an expansion in the B-cell follicle area was observed in the conventional, microbially colonised animals, which was not seen in the germ-free piglets. Consistent with this observation, sequencing of re-arranged heavy chain V-D-J units demonstrated accumulation of point mutations indicating somatic hypermutation in the conventional, microbially colonised piglets but not in the germ-free animals. However, V(H)- and D(H)-segment usage and CDR3 length did not differ between the groups. The results suggest that the follicle reaction observed occurs in response to microbial challenge, involves proliferation and somatic hypermutation of B-cells but does not expand repertoire or generate classical, isotype-switched memory B-cells. We suggest that microbial colonisation of neonatal piglets drives immunological competence in two stages: first, an antigen non-specific, follicular reaction which expands immunological compartments; and second, microbe driven changes in V-segment usage which expand immunological repertoire.

Interactions between Streptococcus suis serotype 2 and cells of the myeloid lineage in the palatine tonsil of the pig.

Streptococcus suis is a major pathogen in pigs and causes significant morbidity and mortality in herds world-wide. A major problem with S. suis is the presence of asymptomatic carrier animals which can spread the organism within and between herds. The palatine tonsil is one of the main sites where the organism can be recovered, both in infected and carrier animals. The use of multiple-colour immunohistology allowed identification of the cell types associated with bacteria in the tonsils of infected gnotobiotic piglets. Bacteria were never associated with T-cells or B-cells but were always associated with cells of the myeloid lineage. Expression of CD16 and CD163 on these leukocytes suggested an association with mature macrophages in tonsil, which may lead to clearance or control of the micro-organism.

Development of the palatine tonsil in conventional and germ-free piglets.

Palatine tonsils, like the Peyer's patches, are considered to be major inductive sites for the mucosa-associated lymphoid tissue (MALT), providing sampling and effector functions for the upper respiratory tract. Consistent with this, they have the architecture required of a classic inductive site (B-cell follicles, immunoglobulin class switching and the presence of naïve and memory T-cells). Here we show that much of this architecture develops after birth in the neonatal piglet, the numbers of T-cells, B-cells and accessory cells increasing with age. Conventional piglets also had higher levels of activated and memory T-cell subsets than germ-free piglets, consistent with development occurring as a result of microbial stimulus. The results suggest that the microbial environment influences the development of the tonsil immunological architecture. Given the role of the tonsil in induction of mucosal responses, this raises questions as to the effectiveness of the tonsil in dealing with colonising organisms in the neonate.