Monomerization of dimeric IgG of intravenous immunoglobulin (IVIg) increases the antibody reactivity against intracellular antigens

Intravenous immunoglobulin (IVIg) preparations are derived from pooled plasma from up to 60,000 healthy human donors and reflect the immunologic experience of the donor population. IVIg contains monomeric and dimeric IgG populations which are in a dynamic equilibrium depending on concentration, pH, temperature, donor pool size, time and stabilizers added in order to keep the portion of dimeric IgG below a certain level. In the present study, monomeric and dimeric fractions were isolated by size exclusion chromatography. The dimeric fractions, however, showed a dynamic instability and tended to dissociate. Both dimeric and monomeric IgG fractions were acid treated (pH 4) in order to dissociate the dimeric IgG. Western-blot analysis identified a sub-population of SDS resistant IgG dimers. Furthermore, the reactivities of the fractions were tested against a panel of self- and exo-antigens. There was a marked increase in activity of the dimeric compared to the monomeric IgG fraction against various intracellular self-antigens. Our data indicates that the increased reactivities of pH 4-treated fractions can mainly be attributed to dimer dissociation, as pH 4-treated monomers do not show significantly increased activities against a range of antigens.

Monomeric and dimeric IgG fractions show differential reactivity against pathogen-derived antigens

Polyvalent Ig preparations, derived from the pooled plasma of thousands of healthy donors, contain a complex mix of both 'acquired' and natural antibodies directed against pathogens as well as foreign and self/auto antigens (Ag). Depending on their formulation, donor pool size, etc., liquid Ig preparations contain monomeric and dimeric IgG. The dimeric IgG fraction is thought to represent mainly idiotype-antiidiotype Ab pairs. Treatment of all IgG fractions at pH 4 effectively monomerizes the IgG dimers resulting in separated idiotype-antiidiotype Ab pairs and thus in a comparable F(ab')(2) binding site availability of the different IgG fractions. Previously, we identified an increased anti-self-reactivity within the monomerized dimer fraction. This study addressed if, among the different IgG fractions, an analogous preferential reactivity was evident in the response against different pathogen-derived protein and carbohydrate antigens. Therefore, we assessed the activity of total unseparated IgG, the monomeric and dimeric IgG fractions against antigenic structures of bacterial and viral antigens/virulence factors. All fractions showed similar reactivity to protein antigens except for exotoxin A of Pseudomonas aeruginosa, where the dimeric fraction, especially when monomerized, showed a marked increase in reactivity. This suggests that the production of antiidiotypic IgG antibodies contributes to controlling the immune response to certain categories of pathogens. In contrast, the monomeric IgG fractions showed increased reactivity towards pathogen-associated polysaccharides, classically regarded as T-independent antigens. Taken together, the differential reactivity of the IgG fractions seems to indicate a preferential segregation of antibody reactivities according to the nature of the antigen.

CD4+ T cell help improves CD8+ T cell memory by retained CD27 expression

CD4+ T cell help during the priming of CD8+ T lymphocytes imprints the capacity for optimal secondary expansion upon re-encounter with antigen. Helped memory CD8+ T cells rapidly expand in response to a secondary antigen exposure, even in the absence of T cell help and, are most efficient in protection against a re-infection. In contrast, helpless memory CTL can mediate effector function, but secondary expansion is reduced. How CD4+ T cells instruct CD8+ memory T cells during priming to undergo efficient secondary expansion has not been resolved in detail. Here, we show that memory CTL after infection with lymphocytic choriomeningitis virus are CD27(high) whereas memory CTL primed in the absence of CD4+ T cell have a reduced expression of CD27. Helpless memory CTL produced low amounts of IL-2 and did not efficiently expand after restimulation with peptide in vitro. Blocking experiments with monoclonal antibodies and the use of CD27(-/-) memory CTL revealed that CD27 ligation during restimulation increased autocrine IL-2 production and secondary expansion. Therefore, regulating CD27 expression on memory CTL is a novel mechanism how CD4+ T cells control CTL memory.