Studies of the palatine tonsils in the pig and of their role in the pathogenesis of piglet streptococcal infection

Abstract

h histological study was made on tonsils from porcine foetuses in late gestation up to aged adults to provide a concept of normality as a background to later experimental studies. In late gestation. lymphocytic follicles were frequently observed in the tonsils. !'erminal centres were noted by four weeks of age, thereafter increasing markedly in number, size and activity. The interfollieular lymphoid tissue developed in cellularity, presenting lymphocytes, macro;ha7es, plasma cells and large pyroninophilic cells by one month of age. Germ -free and dibiotic animals of this age showed an essentially similar tonsillar structure. In pigs up to one year of age there was little variation in this pattern but from then on germinal centre activity decreased markedly. Senile changes included atrophy of the branching tonsillar crypt system and an increase in the connective tissue stroma, replacing much of the lymphoid parenchyma.

To define the afferent and efferent pathways to the palatine tonsils, non-infective studies employing vital dyes ,:ere carried out. Intra-vital dye injections failed to illustrate any afferent route to the tonsils from adjacent areas, while confirming the tonsillar efferent lymphatic drainage to the submandibular lymph nodes. Topical application of India ink to the oral surface of the tonsils defined an afferent route through the tonsillar crypt epithelium to the lymphoid parenchyma. Ink was distributed throughout the inter-follicular tissue, occasionally within germinal centres, and later in trabeculae and the capsule from which the efferent lymphatics take origin. Examination by electron-microscoy revealed that the tonsillar crypt epithelial cells participate in phagocytosis.

Infective studies with Streptococcus suis were carried out to investigate the role of the palatine tonsils in the pathogenesis of piglet streptococcal meningitis and arthritis caused by this organism. Application of :'tree. suis to the oral surface of the tonsils produced clinical disease in one of four piglets infected at ten days of age; inapparent infection occurred in the other three in which the organism was confined to the tonsils and their drainage lymph nodes. :'tree. suis or its antigenic breakdown products, specifically identified by immurofluorescert microscopy. were located in the tonsillar crypt lumina, epithelium, interfollicular tissue and, rarely, in germinal centres, of all four infected piglets. Histological examination failed to reveal any abnormalities of the tonsils. In a second experiment, phagocytosis of bacteria by epithelial cells of the tonsillar crypts was demonstrated by electron - microscopy.

Inapparent infection was established in three piglets aged six weeks. The distribution of Stream. suis was essentially similar to that described above except for an exaggerated presence of Strep. suis antigen in the majority of the tonsillar germinal centres. Attempts to identify specific humoral antibody to ;trep. suis in four older pigs infected systemically with the organism proved unsuccessful. Similarly immunofluorescent methods failed to demonstrate specific antibody -containing cells in the tissues of these four pigs and in the spleens of mice given ,:tree. suis intraperitoneally.