Humoral immunity is characterized by the generation of Ab-secreting plasma cells and memory B cells that can more rapidly generate specific Abs upon Ag exposure than their naive counterparts. To determine the intrinsic differences that distinguish naive and memory B cells and to identify pathways that allow germinal center B cells to differentiate into memory B cells, we compared the transcriptional profiles of highly purified populations of these three cell types along with plasma cells isolated from mice immunized with a T-dependent Ag. The transcriptional profile of memory B cells is similar to that of naive B cells, yet displays several important differences, including increased expression of activation-induced deaminase and several antiapoptotic genes, chemotactic receptors, and costimulatory molecules. Retroviral expression of either Klf2 or Ski, two transcriptional regulators specifically enriched in memory B cells relative to their germinal center precursors, imparted a competitive advantage to Ag receptor and CD40-engaged B cells in vitro. These data suggest that humoral recall responses are more rapid than primary responses due to the expression of a unique transcriptional program by memory B cells that allows them to both be maintained at high frequencies and to detect and rapidly respond to antigenic re-exposure.
The aging of tissue-specific stem and progenitor cells is believed to be central to the pathophysiological conditions arising in aged individuals. While the mechanisms driving stem cell aging are poorly understood, mounting evidence points to age-dependent DNA damage accrual as an important contributing factor. While it has been postulated that DNA damage may deplete stem cell numbers with age, recent studies indicate that murine hematopoietic stem cell (HSC) reserves are in fact maintained despite the accrual of genomic damage with age. Evidence suggests this to be a result of the quiescent (G0) cell cycle status of HSC, which results in an attenuation of checkpoint control and DNA damage responses for repair or apoptosis. When aged stem cells that have acquired damage are called into cycle under conditions of stress or tissue regeneration however, their functional capacity was shown to be severely impaired. These data suggest that age-dependent DNA damage accumulation may underlie the diminished capacity of aged stem cells to mediate a return to homeostasis after acute stress or injury. Moreover, the cytoprotection afforded by stem cell quiescence in stress-free, steady-state conditions suggests a mechanism through which potentially dangerous lesions can accumulate in the stem cell pool with age.
Genome-wide studies have identified associations between polymorphisms in the IFN regulatory factor-5 (Irf5) gene and a variety of human autoimmune diseases. Its functional role in disease pathogenesis, however, remains unclear, as studies in Irf5(-/-) mice have reached disparate conclusions regarding the importance of this transcription factor in type I IFN production and antibody responses. We identified a spontaneous genomic duplication and frameshift mutation in the guanine exchange factor dedicator of cytokinesis 2 (Dock2) that has arisen in at least a subset of circulating Irf5(-/-) mice and inadvertently been bred to homozygosity. Retroviral expression of DOCK2, but not IRF-5, rescued defects in plasmacytoid dendritic cell and B-cell development, and Irf5(-/-) mice lacking the mutation in Dock2 exhibited normal plasmacytoid dendritic cell and B-cell development, largely intact type I IFN responses, and relatively normal antibody responses to viral infection. Thus, confirmation of the normal Dock2 genotype in circulating Irf5(-/-) mice is warranted, and our data may partly explain conflicting results in this field.
Memory B cells (MBCs) and long-lived plasma cells (LLPCs) persist after clearance of infection, yet the specific and nonredundant role MBCs play in subsequent protection is unclear. After resolution of West Nile virus infection in mice, we demonstrate that LLPCs were specific for a single dominant neutralizing epitope, such that immune serum poorly inhibited a variant virus that encoded a mutation at this critical epitope. In contrast, a large fraction of MBC produced antibody that recognized both wild-type (WT) and mutant viral epitopes. Accordingly, antibody produced by the polyclonal pool of MBC neutralized WT and variant viruses equivalently. Remarkably, we also identified MBC clones that recognized the mutant epitope better than the WT protein, despite never having been exposed to the variant virus. The ability of MBCs to respond to variant viruses in vivo was confirmed by experiments in which MBCs were adoptively transferred or depleted before secondary challenge. Our data demonstrate that class-switched MBC can respond to variants of the original pathogen that escape neutralization of antibody produced by LLPC without a requirement for accumulating additional somatic mutations.
Genes regulating responses in mammalian cells are often difficult to identify by functional cloning strategies limited to a single round of selection. Here we describe a strategy, cyclical packaging rescue (CPR), which allows rapid recovery and retransmission of retroviral cDNA libraries. CPR can be used not only with immortalized cell lines such as fibroblasts and Jurkat T cells, but also with primary B lymphocytes, which can be maintained only in short-term cultures. CPR allows for multiple rounds of selection and enrichment to identify cDNAs regulating responses in mammalian cells. Using CPR, five cDNAs were functionally cloned, which conferred protection against tumor necrosis factor alpha (TNFalpha)-induced apoptosis in RelA(-/-) fibroblasts. Three of the genes, RelA, cellular FLICE-like inhibitory protein (c-FLIP), and a dominant-negative mutant of TNF receptor 1 arising through CPR afforded strong protection against apoptosis. Two of the genes identified, Dbs and Fas-associated death domain protein (FADD), previously identified as a proapoptotic molecule, afforded partial protection against TNFalpha-induced apoptosis. These results suggest that CPR is a versatile method that permits functional identification of both wild-type and dominant-negative gene products that regulate cellular responses.
