Both the results presented here as well as those described in earlier studies confirm that CH1 cells represent yet another good model system for recapitu lating BCR driven responses in immature B cells. First, similar to immature and transition stage immature B cells, CH1 cells also express high levels of the IgM class of the BCR, with little or no expression of those belong ing to the IgD class. In immature B cells, BCR activated cells fail to enter into the S phase and this effect can be reversed by treatment with IL4. As we have previously shown, CH1 cells also exhibit similar properties. BCR activation shows contrasting effects on p27 expression in mature versus immature B cells. Mature B cells express high levels of p27, which is then downregulated by antigenic stimulation.

The situa tion is reversed in the case of immature B cells where, while the basal levels of this protein are low, BCR engagement leads to rapid upregulation. As shown in this study, CH1 cells also accurately recapitulate this latter situation. In transitional immature B cells, anti genic stimulation leads to a transient activation of the downstream signaling components including that of Akt/PKB and those belonging to the MAP kinase path way. This feature was also evident in our present examination of BCR signaling in CH1 cells. Finally, the greater extent of ERK phosphorylation relative to that of JNK and p38 observed here was yet another property that is characteristic of antigen stimulated immature B cells.

Thus these comparisons collectively confirm the suitability of CH1 cells as a model for studying mechanisms regulating BCR induced cell cycle arrest and subsequent apoptosis in immature, transitional stage, B lymphocytes. An important aspect of our present study was the sys tems approach that we adopted, which integrated exten sive experimentation with graph theoretical analysis and mathematical modeling. It was the synthesis of these diverse methodologies that enabled us to eventually obtain a comprehensive view on both the quantitative and qualitative features of the BCR dependent signaling network. In addition it also facilitated a description of the consequent AV-951 changes in the transcription regulatory machinery, and the downstream effects on changes in expression levels of those genes that eventually contribu ted towards enforcing a G1 phase specific arrest of the cell cycle. Of particular note here was our finding that the cellular response was, in all likelihood, a direct conse quence of the selective and transient activation of the BCR signaling network. Thus, of the twenty molecules examined, we were only able to observe BCR dependent phosphorylation for fourteen, with no significant effects being evident for the remaining six molecules.