To date, there is no consensus regarding the influence of different CD45 isoforms during peripheral B-cell development. Examining correlations between surface CD45RO expression and various physiologic processes ongoing during the germinal center (GC) reaction, we hypothesized that GC B cells, like T cells, that up-regulate surface RO should progressively acquire phenotypes commonly associated with activated, differentiating lymphocytes. GC B cells (IgD(-)CD38(+)) were subdivided into 3 surface CD45RO fractions: RO(-), RO(+/-), and RO(+). We show here that the average number of mutations per IgV(H) transcript increased in direct correlation with surface RO levels. Conjunctional use of RO and CD69 further delineated low/moderately and highly mutated fractions. Activation-induced cytidine deaminase (AID) mRNA was slightly reduced among RO(+) GC B cells, suggesting that higher mutation averages are unlikely due to elevated somatic mutation activity. Instead, RO(+) GC B cells were negative for Annexin V, comprised mostly (93%) of CD77(-) centrocytes, and were enriched for CD69(+) cells. Collectively, RO(+) GC B cells occupy what seems to be a specialized niche comprised mostly of centrocytes that may be in transition between activation states. These findings are among the first to sort GC B cells into populations enriched for live mutated cells solely using a single extracellular marker.

Tobacco rattle virus (TRV-K) was first identified in a symptomatic Gladiolus plant cultivated in Korea. We analyzed the TRV-K genome and compared its phylogeny with other TRV isolates. After constructing of a full-length genomic RNA2 strand clone, a complete sequence was generated from several overlapping clones. The cloned genome was 3261 bases in length, identical to TRV-K, and had three open reading frames. TRV-K had the highest sequence identity with the American isolate TRV-ORY. Sequence analysis of the RNA2 genome showed that TRV-K contains an intact 2a, 2b, and 2c coding sequence and an RNA1-related 3′ terminus, which is typical of TRV RNA2. Phylogenetic analysis revealed that TRV-K is in the same cluster as the American isolates and another Korean isolate, TRV-SK; however, it was in a different cluster than the European isolates.