Quantitative Contribution of IL2Rγ to the Dynamic Formation of IL2-IL2R Complexes.

Interleukin-2 (IL2) is a growth factor for several immune cells and its function depends on its binding to IL2Rs in the cell membrane. The most accepted model for the assembling of IL2-IL2R complexes in the cell membrane is the Affinity Conversion Model (ACM). This model postulates that IL2R receptor association is sequential and dependent on ligand binding. Most likely free IL2 binds first to IL2Rα, and then this complex binds to IL2Rß, and finally to IL2Rγ (γc). However, in previous mathematical models representing this process, the binding of γc has not been taken into account. In this work, the quantitative contribution of the number of IL2Rγ chain to the IL2-IL2R apparent binding affinity and signaling is studied. A mathematical model of the affinity conversion process including the γ chain in the dynamic, has been formulated. The model was calibrated by fitting it to experimental data, specifically, Scatchard plots obtained using human cell lines. This paper demonstrates how the model correctly explains available experimental observations. It was estimated, for the first time, the value of the kinetic coefficients of IL2-IL2R complexes interaction in the cell membrane. Moreover, the number of IL2R components in different cell lines was also estimated. It was obtained a variable distribution in the number of IL2R components depending on the cell type and the activation state. Of most significance, the study predicts that not only the number of IL2Rα and IL2Rß, but also the number of γc determine the capacity of the cell to capture and retain IL2 in signalling complexes. Moreover, it is also showed that different cells might use different pathways to bind IL2 as consequence of its IL2R components distribution in the membrane.

Human rotavirus-specific IgM Memory B cells have differential cloning efficiencies and switch capacities and play a role in antiviral immunity in vivo.

Protective immunity to rotavirus (RV) is primarily mediated by antibodies produced by RV-specific memory B cells (RV-mBc). Of note, most of these cells express IgM, but the function of this subset is poorly understood. Here, using limiting dilution assays of highly sort-purified human IgM(+) mBc, we found that 62% and 21% of total (non-antigen-specific) IgM(+) and RV-IgM(+) mBc, respectively, switched in vitro to IgG production after polyclonal stimulation. Moreover, in these assays, the median cloning efficiencies of total IgM(+) (17%) and RV-IgM(+) (7%) mBc were lower than those of the corresponding switched (IgG(+) IgA(+)) total (34%) and RV-mBc (17%), leading to an underestimate of their actual frequency. In order to evaluate the in vivo role of IgM(+) RV-mBc in antiviral immunity, NOD/Shi-scid interleukin-2 receptor-deficient (IL-2Rγ(null)) immunodeficient mice were adoptively transferred highly purified human IgM(+) mBc and infected with virulent murine rotavirus. These mice developed high titers of serum human RV-IgM and IgG and had significantly lower levels than control mice of both antigenemia and viremia. Finally, we determined that human RV-IgM(+) mBc are phenotypically diverse and significantly enriched in the IgM(hi) IgD(low) subset. Thus, RV-IgM(+) mBc are heterogeneous, occur more frequently than estimated by traditional limiting dilution analysis, have the capacity to switch Ig class in vitro as well as in vivo, and can mediate systemic antiviral immunity.