A natural hypomorphic variant of the apoptosis regulator Gimap4/IAN1.

The Gimap/IAN family of GTPases has been implicated in the regulation of cell survival, particularly in lymphomyeloid cells. Prosurvival and prodeath properties have been described for different family members. We generated novel serological reagents to study the expression in rats of the prodeath family member Gimap4 (IAN1), which is sharply up-regulated at or soon after the stage of T cell-positive selection in the thymus. During these investigations we were surprised to discover a severe deficiency of Gimap4 expression in the inbred Brown Norway (BN) rat. Genetic analysis linked this trait to the Gimap gene cluster on rat chromosome 4, the probable cause being an AT dinucleotide insertion in the BN Gimap4 allele (AT(+)). This allele encodes a truncated form of Gimap4 that is missing 21 carboxyl-terminal residues relative to wild type. The low protein expression associated with this allele appears to have a posttranscriptional cause, because mRNA expression was apparently normal. Spontaneous and induced apoptosis of BN and wild-type T cells was analyzed in vitro and compared with the recently described mouse Gimap4 knockout. This revealed a "delayed" apoptosis phenotype similar to but less marked than that of the knockout. The Gimap4 AT(+) allele found in BN was shown to be rare in inbred rat strains. Nevertheless, when wild rat DNA samples were studied the AT(+) allele was found at a high overall frequency ( approximately 30%). This suggests an adaptive significance for this hypomorphic allele.

Constitutive expression of the pre-TCR enables development of mature T cells.

Expression and signalling through the pre-TCR and the TCRalphabeta resemble two critical checkpoints during T cell development. We investigated to which extent a pre-TCR can functionally replace mature TCRalpha chains during T cell development. For this purpose, transgenic mice were generated expressing the pre-TCRalpha (pTalpha) under the transcriptional control of TCRbeta regulatory elements. We report here on the interesting finding that constitutive pTalpha expression allows complete T cell maturation. The pre-TCR complex permits a subset of beta-selected thymocytes to mature in the absence of TCRalpha into peripheral T cells (betaT cells) comprising up to 10% of all lymphocytes. Lymphopenia-driven proliferation of these betaT cells is similar to that of conventional alphabetaT cells. Furthermore, betaT cells proliferated and acquired effector function upon stimulation with allogeneic MHC.

Gimap4 accelerates T-cell death.

Gimap4, a member of the newly identified GTPase of the immunity-associated protein family (Gimap), is strongly induced by the pre-T-cell receptor in precursor T lymphocytes, transiently shut off in double-positive thymocytes, and reappears after TCR-mediated positive selection. Here, we show that Gimap4 remains expressed constitutively in the cytosol of mature T cells. A C-terminal IQ domain binds calmodulin in the absence of calcium, and conserved PKC phosphorylation motifs are targets of concanavalin A (ConA)- or PMA/ionomycin-induced PKC activation. To address the role of Gimap4 in T-cell physiology, we completed the genomic organization of the gimap4 locus and generated a Gimap4-null mutant mouse. Studies in these mice revealed no critical role of Gimap4 in T-cell development but in the regulation of apoptosis. We have found that Gimap4 accelerates the execution of programmed cell death induced by intrinsic stimuli downstream of caspase-3 activation and phosphatidylserine exposure. Apoptosis directly correlates with the phosphorylation status of Gimap4.

Visualizing the course of antigen-specific CD8 and CD4 T cell responses to a growing tumor.

Spontaneous tumors frequently express antigens that can be recognized by the immune system but nevertheless manage to evade immune surveillance. To better understand the mechanism of evasion, we followed CD8 and CD4 T cells reacting against a subcutaneously growing tumor, modified to express influenza hemagglutinin (HA) as surrogate tumor antigen. Adoptive transfer of 8,000 antigen-specific CD8 T cells was sufficient to protect against challenge with 1x10(6) tumor cells, while larger numbers of T cells rejected established tumors. HA-specific CD4 T cells could not reject tumors on their own but helped rejection by CD8 T cells. Rejection of the tumor coincided with prolonged survival of expanded antigen-specific CD8 and CD4 T cells, while a failing anti-tumor response was accompanied by transient expansion followed by rapid elimination of antigen-specific T cells. Thus, a highly immunogenic tumor can evade immune surveillance because of an insufficient number of tumor-specific T cells and antigen overload, resulting in exhaustion of the immune response. In this scenario, adoptive immunotherapy rather than vaccination promises successful treatment.