in the absence of costimulation. Two different mechanisms have been proposed for this dramatic effect:
Increased transactivation of the IL-2 gene. The initial signal transduction events are not clear, but it appears that the CD28 cytoplasmic tail will bind to PI-3 kinase after tyrosine phosphorylation. There is also one controversial report of acidic sphingomyelinase activity, leading (probably through several steps) to the activation of Jun N-terminal kinase.
Stability of the IL-2 message. This mechanism for augmenting IL-2 production has been well studied in the mouse and human systems. Early studies postulated that sequences in the 3-prime-1 translated region were important for message stability, but other studies are underway to identify additional critical factors.
Message stability has an effect on the duration as well as the intensity of the response. When the T-cell was stimulated with signal 1 alone (e.g., anti-TCR), the increase in IL-2 message peaks at 4 hours and then falls off fairly rapidly. When the T-cell also received signal 2 (e.g., anti-CD28), the IL-2 message was more prolonged, and there was about a 35-fold difference in the amount of IL-2 produced. Because of this, some researchers believe that message stability rather than transactivation is the dominant effect.
Studies with Knockout Mice. Researchers have also studied costimulation in vivo using CD28 and CTLA4 knockout mice. When they knocked out the CTLA4 gene, which provides the negative feedback loop, they observed massive lymphoproliferation and death at about 3 weeks as lymphocytes infiltrate and destroy multiple organs. These were the effects that might be expected in the absence of a negative feedback. There was some indication that it was actually cardiac problems that killed the mice.
When researchers knocked out the gene for CD28, however, the data were more ambiguous. IgG antibody responses were impaired, but there were normal cytotoxic responses to L, C, and V viruses. Subsequent studies showed impaired cytotoxic responses to VSV viruses, and a measurable but minor impairment of CD4 proliferative responses. These differences were not as dramatic as would be expected, were CD28 the key molecule in costimulation. This has led to further studies in search of the “missing component.”
Other Costimulatory Molecules. Given that the CD28-B7 mechanism does not appear to be the whole story, researchers’ attention will probably turn to additional biochemical mechanisms for costimulation. Among the ligands and receptors listed in Table 3 (above), the cell adhesion molecules such as LFA-1 and ICAMs are important because they allow the cells to come together, and adding antibodies to LFA-1 will block the initiation of T-cell response. The LFA-3-CD2 combination also appears to augment TCR signalling. Other outliers have been known for years, including heat-stable antigen (HSA) and the invariant chain (Ii-c.s.).