We all know, now, that the proteasome is the most important factor in generating peptides for MHC class I. (See the antigen processing overview here if you want a quick review.) There are lots of lines of evidence pointing to this conclusion, but one of the early clues was the presence of interferon-inducible proteasome subunits in the MHC region of the genome.
It turns out (to make a long story short) that proteasomes have at least two different options for their catalytic subunits, and one of these options uses three interferon-inducible subunits. Two of the three are MHC-encoded (LMP2 and LMP7); the third is encoded outside the MHC (MECL1). The twin red flags of genomic organization and response to inflammation suggested that they might have a role in MHC class I antigen presentation; biochemical analysis suggested that the proteasome that incorporated these subunits turned out peptides that are better suited for MHC class I antigen presentation.1 (MECL1 identification trailed the other two by a few years2 and by the time it was shown to be interferon-regulated, some of the excitement was over.) The next step was to make knockout mice lacking each of the subunits.
The LMP2 and LMP7 knockout mice were duly made3 and sure enough both showed (rather modest) phenotypic changes that could be ascribed to changes in MHC class I antigen presentation. MECL1 knockout mice weren’t made until quite recently (as I say, the excitement had sort of passed on to other things by then); and these knockout mice, too, showed T cell anomalies that could be ascribed to antigen presentation changes.4
5 Not so fast. Double knockout mice, lacking both MECL1 and LMP7, are dysfunctional in a completely unexpected way: the double knockout T cells (but not either of the single knockout LMP7 -/- or MECL1 -/- cells) are hyperproliferative, overresponding to some signals. 6 What’s more, this applies to not only CD8 T cells (which one imagines might be affected by MHC class I antigen oddities) but also to CD4 T cells, which shouldn’t much care about MHC class I antigen presentation.A simple straightforward story (in hindsight), right?
An even more recent paper, still in press,7 analyzes this further with chimeric mice. It turns out that even when the T cells mature in the same environment (in other words, when MHC class I antigen presentation is held constant) the MECL1-knockout T cells are still different:
On the other hand, we provide evidence that the effects of MECL-1 on CD4 or CD8 T cell expansion are entirely unrelated to its role in antigen processing. Our findings suggest that MECL-1 influences the homeostatic regulatory processes that maintain the relative proportions of both T cell subsets, through a T cell-intrinsic mechanism independent from thymic or lymphoid interaction partners.
It seems likely that MECL1 somehow affects T cell regulation through affecting some signaling pathways.Â This isn’t completely out of the blue — the proteasome has effects on signaling in several systems (in fact the proteasome affects just about every aspect of cell biology, one way or another). And it’s somewhat reassuring that so far there’s no indication that LMP2 or LMP7 might have caused their effects through anything other than antigen presentation. Still, it’s a warning signal not to take the easy explanation every time.
As a side note, it also makes me wonder about the thymus-specific subunit that was recently identified. When that came out I found it hard to imagine that the very large effects associated with the knockout of this subunit could be explained by antigen presentation. Now I wonder even more, because signaling is something that could easily explain the size of the effects they saw, though I’m not quite sure if it could explain the nature of the effects.
- There are a bunch of references for this, but I’m going to be lazy and just include a couple that I don’t have to think about: Gaczynska, M., Rock, K. L., and Goldberg, A. L. (1993). Gamma-interferon and expression of MHC genes regulate peptide hydrolysis by proteasomes. Nature 365, 264-267.
Gaczynska, M., Rock, K. L., Spies, T., and Goldberg, A. L. (1994). Peptidase activities of proteasomes are differentially regulated by the major histocompatibility complex-encoded genes for LMP2 and LMP7. Proc Natl Acad Sci U S A 91, 9213-9217.[↩]
- Groettrup, M., Kraft, R., Kostka, S., Standera, S., Stohwasser, R., and Kloetzel, P. M. (1996). A third interferon-gamma-induced subunit exchange in the 20S proteasome. Eur J Immunol 26, 863-869.
Hisamatsu, H., Shimbara, N., Saito, Y., Kristensen, P., Hendil, K. B., Fujiwara, T., Takahashi, E., Tanahashi, N., Tamura, T., Ichihara, A., and Tanaka, K. (1996). Newly identified pair of proteasomal subunits regulated reciprocally by interferon gamma. J Exp Med 183, 1807-1816.[↩]
- Van Kaer, L., Ashton-Rickardt, P. G., Eichelberger, M., Gaczynska, M., Nagashima, K., Rock, K. L., Goldberg, A. L., Doherty, P. C., and Tonegawa, S. (1994). Altered peptidase and viral-specific T cell response in LMP2 mutant mice. Immunity 1, 533-541.
Fehling, H. J., Swat, W., Laplace, C., Kuhn, R., Rajewsky, K., Muller, U., and von Boehmer, H. (1994). MHC class I expression in mice lacking the proteasome subunit LMP-7. Science 265, 1234-1237.[↩]
- Basler, M., Moebius, J., Elenich, L., Groettrup, M., and Monaco, J. J. (2006). An altered T cell repertoire in MECL-1-deficient mice. J Immunol 176, 6665-6672.[↩]
- Especially because I am carefully avoiding mention of some of the blind alleys and red herrings that were puzzling at the time.[↩]
- Caudill, C. M., Jayarapu, K., Elenich, L., Monaco, J. J., Colbert, R. A., and Griffin, T. A. (2006). T cells lacking immunoproteasome subunits MECL-1 and LMP7 hyperproliferate in response to polyclonal mitogens. J Immunol 176, 4075-4082.[↩]
- Zaiss, D.M.W., de Graaf, N., & Sijts, A.J.A.M., 2007. The proteasome immunosubunit MECL-1 is a T cell intrinsic factor influencing homeostatic expansion. Infect Immun. doi:10.1128/IAI.01134-07 [↩]