The transcription factor NFκB is a regulator of inflammatory and adaptive immune responses, yet only IκBα has been shown to limit NFκB activation and inflammatory responses. We investigated another negative feedback regulator, IκBε, in regulating B cell proliferation and survival. The loss of IκBε showed increased B cell proliferation and survival in response to both antigenic and innate stimulation. NFκB activity was elevated during late phase activation, but the dimer composition was stimulus-specific. In response to IgM, cRel dimers were elevated in IκBε-deficient cells, yet in response to LPS, RelA dimers were elevated also. The corresponding dimer-specific sequences were found in the promoters of hyper-activated genes. Using a mathematical model of the NFκB signaling system in B cells, we demonstrated that kinetic considerations of the IKK signaling input and IκBε’s interactions with RelA- and cRel-specific dimers could account for this stimulus-specificity. cRel is known to be the key regulator of B cell expansion. We found that RelA-specific phenotype in LPS-stimulated cells was physiologically relevant: unbiased transcriptome profiling identified the inflammatory cytokine, interleukin 6 (IL-6) to be hyper-activated in IκBε−/− B cells. When the IL-6 receptor was blocked, LPS-responsive IκBε−/− B cell proliferation was specifically reduced to near wild type levels. Our results provide novel evidence of a critical role of immune-response functions for IκBε in B cells; it regulates proliferative capacity via at least two mechanisms involving cRel and RelA-containing NFκB dimers. This study illustrates the importance of kinetic considerations in understanding the functional specificity of negative feedback regulators.

The purpose of these studies was to determine the minimal requirements to induce granzyme B, cytotoxic granules and perforin-dependent lytic capacity. To our surprise, both IL-2 and IL-15 induced not only proliferation, but also profound granzyme B and lytic capacity from CD8 + T cells in the absence of antigen or TCR-stimulation. Mouse splenocytes were incubated with mouse r-IL-2 or r-IL-15 for three days, tested by anti-CD3 redirected lysis and examined for intracellular granzyme B and for T cell activation markers. With 10 −8 M IL-2 or IL-15, there was excellent lytic activity at 1:1 effector to target ratios mediated by T cells from wild type but not from perforin-gene-ablated mice, consistent with multiclonal activation. Lower interleukin concentrations induced less lytic activity. Granzyme B was undetectable on day 0, and greatly elevated on day 3 in CD44 hi CD8 + T cells as detected by flow cytometry. Cytokines alone elevated the granzyme B as much as concanavalin A combined with the cytokines. Some ex vivo CD8 + T cells were CD122 + , as were the cultured granzyme B + cells, thus both populations had low affinity receptors for the interleukins. Only some of the activated cells were proliferating as detected by CFSE labeling. When the cytokines were withdrawn, the cells lost lytic activity within 24 hours and then within the next 24 hours, died. Our results suggest that high concentrations of either IL-2 or IL-15 will activate the lytic capacity and granzyme B expression of many T cells and that antigen recognition is not required.

Programmed cell death (PCD) occurs widely in species from every kingdom of life. It has been shown to be an integral aspect of development in multicellular organisms, and it is an essential component of the immune response to infectious agents. An analysis of the phylogenetic origin of PCD now shows that it evolved independently several times, and it is fundamental to basic cellular physiology. Undoubtedly, PCD pervades all life at every scale of analysis. These considerations provide a backdrop for understanding the complexity of intertwined, but independent, cell death programs that operate within the immune system. In particular, the contributions of apoptosis, autophagy, and necrosis in the resolution of an immune response are considered.

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Perforin, a membrane-permeabilizing protein, is important to T cell cytotoxic action. Perforin has potential to damage the T cell in the endoplasmic reticulum (ER), is sequestered in granules, and later is exocytosed to kill cells. In the ER and after exocytosis, calcium and pH favor perforin activity. We found a novel perforin inhibitor associated with cytotoxic T cell granules and termed it Cytotoxic Regulatory Protein 2 (CxRP2). CxRP2 blocked lysis by granule extracts, recombinant perforin and T cells. Its effects lasted for hours. CxRP2 was calcium stable and refractory to inhibitors of granzyme and cathepsin proteases. Through mass spectrometric analysis of active 50-100 kD proteins, we identified CxRP2 candidates. Protein disulfide isomerase A3 was the strongest candidate but was unavailable for testing; however, protein disulfide isomerase A1 had CxRP2 activity. Our results indicate that protein disulfide isomerases, in the ER or elsewhere, may protect T cells from their own perforin.

Pancreatic lipase-related protein 2 (PLRP2) is induced by IL-4 in vitro in cytotoxic T lymphocyte (CTL) clones and CTLs from immunized wild-type (WT) PLRP2(+/+) are more cytotoxic than PLRP2(-/-) CTLs, suggesting to previous investigators that the lipase PLRP2 might support CTL functions. Here, we further evaluate PLRP2 in CTLs. We found that PLRP2 was optimally induced in splenocytes by 3.5 x 10(-8) M IL-4 by day 6 after activation and was restricted to CD8(+) T cells. PLRP2 mRNA was detected inconsistently (and at low levels) after activation in the presence of IL-2. Cytotoxicity in 4 h (51)Cr assays of WT CTLs was approximately 3-fold the activity of PLRP2(-/-) CTLs cultured with IL-4 and, with IL-2, was unexpectedly approximately 2 fold the activity of PLRP2(-/-) CTLs. Thus, PLRP2 gene ablation affected short-term (perforin-dependent) cytotoxicity, even under the IL-2 conditions. Other variables failed to account for the reduced cytotoxicity. Granzyme B levels, activation markers, and CD8(+) T cell frequencies were similar for WT vs. PLRP2(-/-) CTLs (with either cytokine). Addition of rPLRP2 to IL-4 induced PLRP2(-/-) CTLs (or to cytotoxic granule extracts) failed to increase lysis, suggesting that the missing mediator is more than released PLRP2. Cytotoxicity of WT and PLRP2(-/-) CTLs was similar in 2-day tumor survival assays with IL-4, which can be mediated by perforin-independent mechanisms. We conclude that extracellular PLRP2 lipase is unable to directly augment the cytotoxicity that was lost by PLRP2 ablation and that after reevaluation, the question of what is PLRP2's role in CD8 T cells is still unanswered.

IL-4 induces a lipase, pancreatic lipase related protein 2 (PLRP2), in cytotoxic T lymphocytes (CTLs). Because PLRP2 in semen can mediate lipid-dependent toxicity to sperm, we questioned whether CTL-derived PLRP2 could support similar cytotoxicity towards tumor cells. Recombinant PLRP2 was toxic to P815 tumor cells in 48 hours when lipid and another protein, colipase, were present. However, PLRP2-positive CTLs (induced with many lots of IL-4) were unable to mediate lipid-dependent cytotoxicity. Notably, CTLs induced with only one lot of IL-4 had lipiddependent cytotoxicity. The exceptional lot of IL-4 was effective in multiple experiments at inducing lipid-dependent cytotoxicity. The lipid-dependent cytotoxicity it induced was determined to be perforin-independent. CTLs induced with IL-4 that was unable to induce lipid-dependent cytotoxicity had mRNA for PLRP2 but not mRNA for colipase. Therefore, we added exogenous colipase to the CTL assays but still cytotoxicity was unchanged. We conclude (1) that lipiddependent cytotoxicity, promoted by the lipase PLRP2 and colipase, will kill tumor cells and (2) that more than PLRP2 alone is required for lipid-dependent cytotoxicity mediated by CTLs.

It is widely accepted that naïve T cells require two signals, antigen recognition and co-simulation, to become cytotoxic over the course of 3-5 days. However, we observed that freshly isolated murine splenocytes without exposure to antigen become cytotoxic within 24 hours after culture with IL-15. IL-15 is a cytokine that promotes homeostatic proliferation, maintenance and activation of memory T cells. The induced cytotoxicity, measured by anti-CD3 redirected 51 Cr release, represented the combined activity of T cells regardless of their antigen specificity, and proceeded even when CD44 hi (memory-associated phenotype) CD8 + T cells were depleted. Cytotoxic capacity was perforin-dependent and occurred without detectable up regulation of granzyme B or cell division. After induction, the phenotypic markers for the memory subset and for activation remained unchanged from the expression of resting T cells. Our work suggests that T cells may gain cytotoxic potential earlier than currently thought and even without TCR stimulation.