The archetypal human tumor suppressor p53 is considered to have unique transactivation properties. The assumption is based on the fact that additionally identified human p53 isoforms lack transcriptional activity. However, we provide evidence for the existence of an alternatively spliced p53 isoform (Deltap53) that exerts its transcriptional activity independent from p53. In contrast to p53, Deltap53 transactivates the endogenous p21 and 14-3-3sigma but not the mdm2, bax, and PIG3 promoter. Cell cycle studies showed that Deltap53 displays its differential transcriptional activity only in damaged S phase cells. Upon activation of the ATR-intra-S phase checkpoint, Deltap53, but not p53, transactivates the Cdk inhibitor p21. Induction of p21 results in downregulation of cyclin A-Cdk activity and accordingly attenuation of S phase progression. Data demonstrate that the Deltap53-p21-cyclin A-Cdk pathway is crucial to facilitate uncoupling of repair and replication events, indicating that Deltap53 is an essential element of the ATR-intra-S phase checkpoint.

Current combination antiretroviral therapies (cART) efficiently suppress HIV-1 reproduction in humans, but the virus persists as integrated proviral reservoirs in small numbers of cells. To generate an antiviral agent capable of eradicating the provirus from infected cells, we employed 145 cycles of substrate-linked directed evolution to evolve a recombinase (Brec1) that site-specifically recognizes a 34-bp sequence present in the long terminal repeats (LTRs) of the majority of the clinically relevant HIV-1 strains and subtypes. Brec1 efficiently, precisely and safely removes the integrated provirus from infected cells and is efficacious on clinical HIV-1 isolates in vitro and in vivo, including in mice humanized with patient-derived cells. Our data suggest that Brec1 has potential for clinical application as a curative HIV-1 therapy.

Dendritic cells (DCs) 3 are the most potent antigen-presenting cells of the immune system and are specialized to sensitize helper and killer T cells during the induction of T cell-mediated immunity (1, 2). In fact, DCs are the only antigen-presenting cells that are able to stimulate naive CD4 ϩ and CD8 ϩ T cells and are therefore referred to as "nature's adjuvant."The CD83 molecule is to date the best known marker for fully mature DCs because CD83 is predominantly expressed on the surface of dendritic lineage cells and cannot be detected on immature DC precursors (3-5). Although its exact function remains to be determined, the fact that CD83 expression is activated during DC maturation, together with co-stimulatory molecules such as CD80 and CD86, suggests a functionally important role for CD83 in DC-mediated T cell immunity (6, 7). This notion is also supported by the fact that inhibition of CD83 expression during DC maturation reduces the T cell stimulatory capacity of these DCs in allo-mixed leukocyte reactions (8). A study using the soluble extracellular domain of CD83 has provided the first direct evidence that this specific surface molecule is indeed functionally important for T cell activation; the soluble CD83 protein completely inhibited DC-mediated T cell stimulation in a concentration-dependent manner in vitro (9). These data were subsequently confirmed in an independent study by using CD83-Ig fusion protein (10). Thus, CD83 appears to play an important functional role in the regulation of DC-mediated T cell-specific immune responses. Therefore, the investigation of the regulation of CD83 expression may provide novel opportunities to modulate DC activity and subsequently DC-mediated immune responses.

Dendritic cells (DC) are the most potent antigen-presenting cells of the immune system and are able to sensitize even naïve T cells. Mature DC are characterized by expression of CD83, a surface molecule that is proposed to be involved in efficient T cell activation. It has been recently shown that CD83 mRNA is transported from the nucleus to the cytoplasm in a HuR-and CRM1-dependent manner. Therefore we here investigated the impact of two known protein ligands of HuR, pp32 and APRIL, on CD83 expression. Both pp32 (ANP32A) and APRIL (ANP32B) are shuttle proteins, and it has been reported earlier that these HuR ligands can act as adaptors that link HuR and the CRM1-specific nuclear export pathway. By employing RNA interference (RNAi) technology we demonstrate that pp32 is dispensable for CD83 expression, whereas APRIL contributes to the nuclear export and subsequent translation of CD83 mRNA. Furthermore, we have determined the nuclear import signal (NLS) as well as the nuclear export signal (NES) of human APRIL. Moreover, we analyzed the status of phosphorylation of endogenous APRIL and identified threonine 244 to be an as yet unrecognized phosphate acceptor. Finally, we were able to show that phosphorylation of this specific amino acid residue regulates the nuclear export of APRIL. In sum, we report here the signal sequences in APRIL that mediate its intracellular trafficking and provide evidence that this protein ligand of HuR is an important player in the post-transcriptional regulation of CD83 expression by affecting the nucleocytoplasmic translocation of CD83 mRNA.