Biomedical Engineering Reference
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of cells in the samplemight be responding in a very aberrant manner. In our laboratory,
a study of juvenile myelomonocytic leukemia (JMML) revealed that a rare population
of cancer cells, otherwise masked by a preponderance of normal myeloid cells,
responded to a growth factor with very aggressive signaling [7]. Similarly, in
autoimmune disease, analysis of na
ve T cells and B cells may give an incomplete
picture of what is driving disease, whereas analysis of specific populations of effector
lymphocytes and regulatory T cells will provide a mechanistic understanding of the
disease.
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15.3.2 Mechanistic Complexity of Signaling Changes
When analyzing changes in signaling responses to cytokines, a common question is
whether the level of the cytokine receptor at the cell surface correlates with the
magnitude of signaling observed. Although in some cases there is a measurable
correlation, intracellular factors also play roles in modulating the signaling trans-
duced from the cell surface. Strong cellular responses can come from cells that exhibit
barely detectable levels of receptor on their surface and cells that have high levels of
receptor expression often show muted or limited signaling responses. Therefore, it is
of critical importance to evaluate the actual signaling induced by a particular
cytokines and not to exclusively use receptor expression as a surrogate marker.
Indeed, our investigations of systemic lupus erythematosus show that receptor
expression levels do not directly correlate with the ability of cells to strongly or
weakly transduce cytokine signals from their environment [8]. Starting early in
disease progression, the Stat1 signaling response to IL-6 was greatly reduced in both
CD4 þ and CD8 þ T cells [8]. While downregulation of the IL-6 receptor partially
explained this reduced sensitivity to IL-6 in CD4 þ T cells, expression of the receptor
was unchanged in the CD8 þ T cells and the Stat3 response to this cytokine remained
robust in the CD8 þ subset. This differential activation of Stat1 and Stat3 was evident
in individual cells. We found that the Stat1 arm of the signaling response was greatly
inhibited by the upregulation of SOCS1 protein levels, amolecule known to inhibit the
ability of Jak2 to phosphorylate Stat1. A similar pathway-specific inhibition of Stat1
relative to Stat3 was observed when T cells were stimulated with IL-21. As expected,
the disease-associated inhibition of Jak2 activity was found to reduce the induction of
a number of Stat1 target genes. Therefore, both receptor expression and intracellular
factors played roles in modulating the IL-6 and IL-21 responses, yielding multiple
potential drug targets.
In the MRL/lpr model of SLE, mice progress with great consistency from a
relatively normal immune profile (at 5 weeks of age) through an early phase of
autoimmunity characterized by relatively mild symptoms (at 10-12 weeks of age) on
to more severe pathology, including lupus nephritis (at 15-20 weeks of age). We
examined cytokine signal transduction through Jak-Stat signaling pathways at four
different stages of lupus progression in these mice. At 5 weeks of age, we found that
signaling was relatively normal across all cell types in the study and there was no
significant difference from the control MRL strain. However, starting at 10 weeks of
age, we observed robust changes in the ability of multiple immune cell types from the
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