Human Immunoglobulin G2 (IgG2) and IgG4, but Not IgG1 or IgG3, Protect Mice against Cryptococcus neoformans Infection(1)
As therapeutic agents for infectious disease, antibodies have unique properties, including exquisite specificity, toxin neutralization, receptor blockade, opsonization, complement activation, and antibody-dependent cell-mediated cytotoxicity. The widely held belief is that human IgG1 has the most favorable constellation of properties since it binds complement and all three classes of cellular receptors for IgG (FcγRs) and has a long half-life (t1/2). C. neoformans elaborates a thick polysaccharide capsule primarily composed of glucuronoxylomannan (GXM). Passively administered MAbs to GXM can prolong survival of infected mice; FcγR binding and complement activation appeared to play a role in antibody efficacy.
Fifty percent of the maximal optical density (EC50) was observed with 3.1 to 3.8 ng of 3E5 IgG1, 3E5 IgG2, and 3E5 IgG3. The relative affinity of 3E5 IgG4 (EC50, 10.1 ng) was somewhat lower than those of the other isotypes. Monomeric IgG4 has two antigen binding sites, while IgG4 half molecules have one binding site.
BALB/c mice were given 1 mg IgG i.p. and then infected intravenously with C. neoformans 18 h later. In comparison to isotype-matched and PBS controls, IgG2 and IgG4 anti-GXM prolonged the survival of infected mice. Recombinant IgG3 anti-GXM was no different than IgG3 anti-dansyl. Treatment with IgG1 anti-GXM shortened the lives of mice infected with C. neoformans.
A prozone effect with mouse IgG MAb treatment of i.p. cryptococcal infection. Less than 1 mg of IgG1 did not significantly affect mouse survival compared to PBS, indicating that a prozone effect was not responsible for the apparent enhancement of infection seen at the 1-mg dose of IgG1. In addition, 1 mg IgG2 was most efficacious, with less protection seen with the lower doses.
mice treated with all four 3E5 IgG subclasses had decreased levels of GXM. This effect was most pronounced in mice with 3E5 IgG2. While GXM levels in mice treated with IgG1, IgG3, and IgG4 were similar, only IgG4 provided protection. In general, MAHA titers were low (≤1:200) to undetectable. Twelve of 88 mice treated with anti-GXM human IgG had MAHA titers of >1:200, with 5 mice showing titers equal to 1:800. There was no correlation between MAHA titer and isotype or survival from cryptococcal infection. The half-lives of antibodies were somewhat longer than those previously observed for anti-dansyl mouse-human chimeric IgGs produced in a similar mannerThe overall hierarchy was preserved, with IgG2 having a very long t1/2 and IgG4 having the shortest t1/2.
In mouse C1q labeled with FITC and flow cytometry, both nonprotective isotypes, IgG1 and IgG3, had detectable C1q binding, while the protective antibodies, IgG2 and IgG4, did not.
Mouse IgG1 prolonged the life of A/J, BALB/c, and C57BL6/J mice infected with C. neoformans, while IgG3 did not and, in some cases, mice treated with this isotype died earlier than PBS-treated mice. IgG3 differs from the other mouse IgG subclasses in that it forms aggregates and binds FcγRI but not FcγRII or FcγRIII. The protective mouse isotypes (IgG1, IgG2b, and IgG2a) all bind mouse FcγRII and FcγRIII but differ in their abilities to activate complement. That human IgG1 was disease enhancing in this study looking at BALB/c mice but was found to be protective in A/J mice, which are C5 deficient, suggests that complement may play a role in MAb-mediated enhancement of cryptococcal infection.