Nanobiologics: a real game changer for targeted immunotherapy

Organ transplantation has proven to be very effective in a variety of end-stage diseases. Immunosuppressive agents prescribed to patients modestly improves graft survival, but long term use can have detrimental side effects, leaving patients with increased risk of infection, cancer, and metabolic toxicity. Strategies targeting adaptive immune cell tolerance induction have proven promising, however, long-term graft survival rates still remain sub-optimal. More recently innate immune cells, including macrophages, NK cells, and monocytes have been identified as key players in the initiation of allograft rejection. The mechanisms by which macrophages mediate graft loss remain poorly understood. Recently, a study by Braza et al., identified a macrophage activation pathway linked to allograft rejection. The authors utilized a novel myeloidspecific nanoimmunotherapy to target graft-infiltrating macrophages to promote long-term organ transplant acceptance. Proinflammatory activation of macrophages by danger-associated molecular patterns (DAMPs) vimentin and high-mobility group box 1 (HMGB1) by dectin-1 and TLR4 activation has been previously shown. Using an experimental heart transplantation mouse model, Braza et al., found both vimentin and HMGB1 were up-regulated in allograft infiltrating macrophages; raising the question whether these targets could promote ‘trained immunity’. Innate immune cells can acquire trained immunity or ‘innate immune memory’, whereby secondary non-specific challenge, after pre-exposure to certain inflammatory stimuli triggers epigenetic and metabolic changes, enhancing pro-inflammatory responses. Monocytes pre-exposed to vimentin, followed by re-stimulation with HMGB1, increased pro-inflammatory IL-6, and TNFa production in vitro. Graft-infiltrating macrophages from dectin-1 and TLR4 deficient mouse heart allografts produced significantly lower levels of IL-6 and TNFa following ex vivo stimulation. To target trained macrophages therapeutically, Braza et al., utilized myeloid-specific high-density lipoproteins (HDL) nanobiologics. The authors armed their nanobiologics with rapamycin, an mTOR inhibitor (termed mTORi-HDL), which has been shown to inhibit myeloid cell activation and pro-inflammatory cytokine production. Heart allografts in recipient mice were shown to accumulate mTORi-HDL and its uptake preferentially by myeloid cells, namely macrophages. A treatment regime of three intravenous doses of mTORi-HDL was sufficient to prolong graft survival, compared with placebo control or mice treated with oral rapamycin. Following ex vivo stimulation, it was shown that these macrophages had a marked reduction in pro-inflammatory cytokine production, suggesting trained macrophage responses were impaired because of mTORi-HDL treatment. Recipient allografts, blood and spleen were shown to have increased numbers of Ly6C macrophages, reported to have anti-inflammatory properties. The authors confirmed that Ly6C macrophages inhibited T-cell proliferation and promoted regulatory T cell (Treg) expansion in vitro, in addition, mTORi-HDL allografts were shown to have significantly elevated Tregs counts. Irrespective of mTORi-HDL treatment, Ly6C macrophage depletion prior to transplantation resulted in early graft rejection. Allograft survival was restored following adoptive transfer of wild-type monocytes, reinforcing the view that the mTORi-HDL therapy requires Ly6C regulatory macrophages for successful organ transplant acceptance. Finally, the authors evaluated the impact of combinational therapy with mTORi-HDL and a CD40-CD40L targeting nanbiologic, TRAF6i-HDL. They found that short-term combination therapy with mTORi-HDL/TRAF6i-HDL synergistically promoted long-term allograft survival, and significantly outperformed either monotherapy. The study by Braza et al., clearly highlights the importance of innate immune signalling in driving trained immunity and excessive inflammation. In the context of cardiovascular disease, Bekkering et al., found that exposing monocytes to a low concentration of oxLDL drove a trained proatherogenic macrophage phenotype characterized by increased pro-inflammatory cytokine production and foam cell formation. A review by Zhang et al. emphasized the importance of trained macrophage activation pathways in pathological conditions such as cardiac remodelling and ischaemic heart disease. A recent review by Kusters et al. reiterated the importance of immune checkpoint molecules and their potential as therapeutic targets in atherosclerosis. The concept of targeting immune checkpoint molecules, such as CD40-CD40L, as chosen by Braza et al., or others, like CD80/86-CD28, is not a novel concept in of itself. Most experimental small molecules for atherosclerosis have failed in clinical trials due to their unfavourable toxicity profiles, generally due to the high accumulation in non-targeted tissue or non-targeted cells within the targeted tissues. However, the use of nanobiologics to successfully administer high local doses of immunosuppressive agents, while