Germinal center T cells exhibit properties of memory helper T cells

Potential roles of germinal center (GC) T cells in immune memory were investigated by examining T cells in GCs lymph nodes (LNs) following immunization with myelin basic protein (MBP). MBP preferentially stimulates T cells expressing the V beta 8 TCR family in B10.PL mice. By staining lymph nodes with peanut agglutinin, GCs were first identified 8 days after immunization and could still be detected 50 days later. When T cells within GCs were examined at Day 8, the majority (> 90%) were CD45RB-negative and a large percentage (20%) had already proliferated several days before as determined by bromodeoxyuridine (BrdU) incorporation. When BrdU+ V beta 8+ cells were examined in various areas of LNs 3 and 8 weeks after immunization, BrdU+ V beta 8+ cells were approximately fourfold more frequent in GCs than other areas of LNs. When GCs were examined for IL-4 mRNA+ cells using an in situ hybridization method, only 0.01% of GC cells expressed IL-4 mRNA, corresponding to about 1% of GC T cells. Shortly following restimulation with MBP, the frequency of cells expressing IL-4 mRNA increased fivefold. Taken together, these findings indicate that GC T cells possess characteristics associated with memory T cells.