T Cell Dependent Antigen-Induced Immunoglobulin Isotype Swiching and Diifferentiation of Lymph Node

Kim, Joong Gon; Lee, Wang Jae

Abstract

Lymph nodes, one of peripheral lymphoid organs, are the sites, where the lymphocytes receive their initial instructions for producing effector functioning resulting in humoral or cell-mediated immunity. Each lymph node consists of an outer cortex in which there are aggregates of cells constituting the follicles, B-cell areas. Some follicles have central areas called germinal centers, which stain lightly. Germinal centers are B lymphoblast cell areas arising eccentrically in primary lymphoid follicles in response to T-cell dependent antigenic stimulation and are the generally accepted sites of generation of memory B cells and undergoing isotype switching and somatic mutation. We observed the morphologic, cellular, protein and molecular events arising in mouse popliteal lymph nodes in response to T-cell dependent antigenic stimulation. In this study mice were immunized into footpads with TNP-chicken ovalbumin. The germinal center formation in primary follicles of popliteal lymph nodes was first observed 6 days after immunization and germinal centers persisted until 24 days of immunization. Lymph node cells were stained with PE-labeled anti-B220 antibody and/or FITC labeled PNA and analyzed by using FACScan. B cells (B220(+) cell) in lymph node increased after 3 days and peaked between 6 and 18 days after immunization. The proportion of germinal center B cells (B220, PNA(high) cells) among lymph node B cells was low (2%) before immunization but increased at day 6 (9%) and reached the peak (30%) at day 18. The expression of IgG1 productive mRNAs and germline transcripts were observed by using RT-PCR. The expression of IgG1 productive mRNA was detected at day 10 and continued until 24 days after immunization. The expression of IgG1 germline transcripts was observed 10 days after immunization and rapidly declined over the next one week. IgG1 anti-TNP antibody, main isotype of anti-TNP antibodies, was first detected at day 14 and reached the peak level 24 days after immunization. Taken these data together, we can conclude that the first immunological event observed from mouse popliteal lymph node in response to T-cell dependent antigenic stimulation is the increase in the number of B cells, and this event is followed by appearance of germinal center B cells and at the same time by the formation of germinal center in primary lymphoid follicles. Once the germinal center is formed, the process of isotype switching to IgG1 occurs in lymph node and antigen-specific IgG1 antibody is produced.