Experiments hold intriguing, circumstantial clues to the mechanisms behind arginine-mediated solubilization of small organic drugs and suppression of protein aggregation driven by hydrophobic or aromatic associations, but how exactly arginine's molecular structure and interactions contribute to its function remains unclear since attention has focused so far on the thermodynamics of the preferential exclusion or binding of arginine. Here, we examine, through molecular dynamics simulations, how arginine solubilizes nanoscale particles with hydrophobic surfaces or aromatic-ring-type surface interactions. We show that preferential, hydrophobic, and dispersion interactions of arginine's guanidinium group with the particles lead to a surfactant-like behavior of arginine around the particles and to a solvation layer with a protective polar mask creating a hydrophilic shell. Additionally, arginine-arginine association around the solvation layer further prevents aggregative contacts. The results shed some light on the mechanistic basis of arginine's function as a suppressant of protein aggregation, although the complex energy landscapes and kinetic pathways of aggregation are protein-dependent and pose formidable challenges to developing comprehensive mechanistic pictures. Our results suggest arginine's mode of interaction with hydrophobic patches and aromatic residues could reduce aggregation-prone intermediate states of proteins and shield protein-protein aggregative contacts. The approach used here offers a systematic way of exploring implications of other amino acid/excipient interactions by studying interactions of the excipient with particles grafted with amino acids.

A murine model system was established to study immune responses to the Pnu-Imune vaccine, which is made up of 23 different pneumococcal capsular polysaccharides. In this animal model, antibody-forming cell responses to 21 of 23 individual polysaccharides in the vaccine were detected. The Pnu-Imune vaccine elicited good antibody responses from the spleens and mesenteric lymph nodes (MLN) of young mice, whereas a variety of other peripheral lymph nodes were unresponsive. The immunoglobulin M plaque-forming cell (PFC) response in the spleen to the Pnu-Imune vaccine (given intraperitoneally or subcutaneously) decreased dramatically with increasing age. However, the spleen and MLN differed in their susceptibility to an age-associated decline in immune function. While the PFC responses in the spleen declined with age, the PFC response in the mucosa-associated MLN did not decline with age but instead remained constant over the entire age span of 4 to 28 months studied. These studies showed that the spleen, peripheral lymph nodes, and MLN did not demonstrate parallel age-associated defects in antibody responses to pneumococcal polysaccharides when the antigen was administered systemically. Also, the deficient splenic antibody response to Pnu-Imune vaccine in aged mice could be enhanced by injecting a combination of Pnu-Imune vaccine and the nontoxic adjuvant monophosphoryl lipid A. Moreover, an immunoglobulin G response was induced when the immunogen was a mixture of vaccine and adjuvant.

Previously, model systems were developed in our laboratory to study murine immune responses to the 23-valent pneumococcal polysaccharide vaccine Pnu-Imune, both in vivo and in vitro (M. Garg and B. Subbarao, Infect. Immun. 60:2329-2336, 1992; M. Garg, A. M. Kaplan, and S. Bondada, J. Immunol. 152: [1589][1590][1591][1592][1593][1594][1595][1596] 1994). Using these systems, we found that aged mice did not respond to the vaccine in vivo or in vitro. Cell separation studies showed that the unresponsiveness of the aged spleen cells to the vaccine was not due to an intrinsic B-cell defect or to T-cell-mediated immunosuppression but resulted from an accessory cell deficiency. Irradiated spleen cells from young mice enabled the old mouse spleen cells to respond to the vaccine. Interestingly, irradiated spleen cells from old mice also restored the vaccine responsiveness in old mice but were required in greater numbers than the young mouse spleen cells to induce similar levels of response. The accessory cell was an adherent cell that could be removed by passage through Sephadex G-10 and thus may be a macrophage. Accessory function could also be provided by the cytokine interleukin-1 (IL-1), IL-4, or IL-5 but not IL-2 or IL-6. Thus, one reason for the deficient immune response to pneumococcal vaccine in aged mice is a quantitative defect in adherent accessory cells.

No abstract

Recently, we reported that murine antibody responses to the 23-valent pneumococcal polysaccharide (Pnu-Imune) vaccine declined with age. Here we present data to support the concept that age-associated immune defects are not only due to intrinsic defects in immune cells but are also due to extrinsic factors emanating from the neuroendocrine system. We found that supplementation with dehydroepiandrosterone, a steroid hormone known to be reduced in the aged, corrects the immune deficiency of aged mice and significantly enhanced their splenic immune responses to the Pnu-Imune vaccine.