Atherosclerosis is a chronic inflammatory disease driven by lipids and particularly by immunogenic oxidation-specific epitopes (OSEs) of oxidated LDL (oxLDL). A number of studies have provided evidence that anti-oxLDL natural IgM antibodies, mostly produced by innate marginal zone (MZ) B cells, are atheroprotective in hyperlipidemic ApoE-/- or Ldlr-/- mice. The production of protective anti-oxLDL IgM antibodies can be boosted in mice by immunization with OSEs displayed by phosphorylcholine (PC), by apoptotic cells, or by killed pneumococci through the polysaccharide-associated PC. The latter finding prompts the appealing possibility of making use of the current pneumococcal polysaccharide vaccines for inducing atheroprotective antibodies in humans but attempts with a 13-valent vaccine (Prevenar) have so far failed to yield robust anti-oxLDL responses. A likely reason for this failure is that human autoreactive MZ B cells continually exposed to autoantigen, as it is the case for anti-oxLDL B cells, largely become anergic ¿CD21low¿ B cells with unresponsive B cell receptors (BCR). Importantly, it is known that the contemporaneous engagement of the BCR and of Toll-like receptors (TLR) 9 or 7 can overcome anergy in these cells; thus, while the immunogens used in mouse studies simultaneously provide OSEs that stimulate the BCR and ligands (bacterial or cellular DNA, CpG) that stimulate TLR9, polysaccharide vaccines used in human studies stimulate only the BCR. Thus, formulations of pneumococcal polysaccharide adjuvated by TLR9 or TLR7 ligands are expected to induce robust, potentially atheroprotective, anti-oxLDL responses.
The objective of the project is to determine the functional status of human oxLDL-specific B cells in healthy donors and in patients with hyperlipidemia and diffuse atherosclerosis, and the optimal conditions in vitro of costimulation with TLR9 or TLR7 ligands and pneumococcal polysaccharide for inducing an IgM anti-oxLDL antibody response.
Several anti-inflammatory therapies for atherosclerosis have been attempted and many are under investigation in clinical trials, but it is predictable that high costs and potential hazards would limit a widespread use of many of these therapies even if, like canakinumab (1), they would prove effective. Passive or active immunization using, respectively, atheroprotective antibodies or vaccines that induce an atheroprotective antibody response are appealing strategies because they involve reasonably low risks and, in the latter case, costs. However, vaccine formulations that succeeded in reducing atherosclerosis in pre-clinical models have so far failed translation to the clinic (2). A particularly appealing strategy is vaccination with Streptococcus pneumonaie, which has been shown to induce atheroprotective anti-oxLDL IgM antibodies as a response to the phosphorylcholine mimotope of the bacterial polysaccharide (3,4). The reason(s) for the failure of the immunization with standard pneumococcal polysaccharide vaccines to significantly boost the production of anti-oxLDL IgM antibodies in clinical trials (5) are unclear and may include a low content of phosphorylcholine in the vaccine polysaccharide preparations or inadequacy of the immunization protocols used. However, the fact that successful studies in mice made use of immunogens containing TLR9 agonists, either fortuitously as in the case of bacterial DNA in whole pneumococci or purposely as in the case of phosphorylcholine adjuvanted with CpG, makes it reasonable to guess that a standard pneumococcal polysaccharide vaccine adjuvanted with a TLR9 (or TLR7) agonist should be able to induce a similarly robust production of anti-oxLDL IgM antibodies in humans. Having this in mind, we design preclinical studies making use of TLR adjuvants already approved for clinical trials, so that it will be possible to directly translate results to a proof of the concept small trial in humans. The success of the preclinical and clinical phases of this project, besides representing a significant scientific breakthrough, would open the way to clinical trials investigating the potential protective effects of adjuvanted pneumococcal vaccination on CVD. If confirmed by such studies, atheroprotection with adjuvanted pneumococcal vaccine could represent a cost effective and safer alternative to many of the therapies currently in clinical investigation (eg canakinumab, anakinra, tocilizumab, interleukin-2, xilonix and other drugs). Concerning safety, the TLR9 agonist CPG 7909 demonstrated an excellent profile in several clinical studies, and also enhanced antibody response to pneumococcal vaccine in HIV-infected adults (6). Thus, a CPG 7909-adjuvanted formulation of the current pneumococcal polysaccharide vaccine(s) could in principle provide both improved protection from infection and atheroprotection through the boosting of anti-oxLDL IgM responses in subjects at risk for CVD, with a potentially significant impact on public health.
1. Ridker PM. N. Engl. J. Med 2017;377:1119¿31
2. Zhao TX. J Am Coll Cardiol 2019;73:1691-1706
3. Binder CJ et al. Nat Med. 2003 Jun;9(6):736-43
4. Caligiuri G. et al. J. Am. Coll. Cardiol. 2007;50:540-6
5. Grievink HW et al. 2020 Oct 22;9(11):345
6. Søgaard OS et al. Clin Infect Dis. 2010 Jul 1;51(1):42-50