Germinal center B cells in Peyer's patches of aged mice exhibit a normal activation phenotype and highly mutated IgM genes.

Systemic and mucosal humoral immune responses have been reported to exhibit an age related decline. However, differences in primary and memory responses at mucosal sites in aged mice have been noted. In an effort to begin characterizing deficiencies in the mucosal system of aged mice, we examined the B cell compartment of gut associated Peyer's patch lymphoid tissue. To our surprise, we found that germinal center (GC) B cells from aged B6D2F1 mice (24-26 months) were present at similar frequencies and exhibited a normal activation phenotype such as upregulation of B7.1, B7.2 and CD44, and downregulation of CD23, CD62L and CD38 as that observed in younger mice (2.5-4 months). As expected, Peyer's patch GC B cells from aged mice expressing V(H)X24 genes displayed higher somatic mutation frequencies compared with younger mice. However, this was particularly striking in IgM sequences where high mutational loads suggested we were sampling memory cells. It is conceivable that B-cells expressing these genes reflect the presence of a mucosal memory compartment in aged mice that either retains flexibility in effector function or is committed to the secretion of IgM antibody.

Expression levels of Mycobacterium tuberculosis antigen-encoding genes versus production levels of antigen-specific T cells during stationary level lung infection in mice.

Mycobacterium tuberculosis lung infection in mice was controlled at an approximately stationary level after 20 days of log linear growth. Onset of stationary level infection was associated with the generation by the host of T helper type 1 (Th1) immunity, as evidenced by the accumulation of CD4 Th1 cells specific for the early secretory antigen (ESAT-6) of M. tuberculosis encoded by esat6, and for a mycolyl transferase (Ag85B) encoded by fbpB. CD4 T cells specific for these antigens were maintained at relatively high numbers throughout the course of infection. The number of CD4 T cells generated against ESAT-6 was larger than the number generated against Ag85B, and this was associated with a higher transcription level of esat6. The total number of transcripts of esat6 increased during the first 15 days of infection, after which it decreased and then approximately stabilized at 10(6.5) per lung. The total number of fbpB transcripts increased for 20 days of infection before decreasing and then approximately stabilizing at 10(4.8) per lung. The number of transcripts of esat6 per colony-forming unit of M. tuberculosis fell from 8.6 to 0.8 after day 15, and of fbpB from 0.3 to less than 0.02 after day 10, suggesting that at any given time during stationary level infection the latter gene was expressed by a very small percentage of bacilli. Expressed at an even lower level was an M. tuberculosis replication gene involved in septum formation (ftsZ), indicating that there was no significant turnover of the M. tuberculosis population during stationary level infection.