17Symbiotic associations impact and are impacted by their surrounding ecosystem. The 18 association between Burkholderia bacteria and the soil amoeba Dictyostelium 19 discoideum is a tractable model to unravel the biology underlying symbiont-endowed 20 phenotypes and their impacts. Several Burkholderia species stably associate with D. 21 discoideum and typically reduce host fitness in food-rich environments while increasing 22 fitness in food-scarce environments. Burkholderia symbionts are themselves inedible to 23 their hosts but induce co-infections with secondary bacteria that can serve as a food 24 source. Thus, Burkholderia hosts are "farmers" that carry food bacteria to new 25 environments, providing a benefit when food is scarce. We examined the ability of 26 specific Burkholderia genotypes to induce secondary co-infections and assessed host 27 fitness under a range of co-infection conditions and environmental contexts. Although all 28Burkholderia symbionts intracellularly infected Dictyostelium, we found that co-infections 29 are predominantly extracellular, suggesting that farming benefits are derived from 30 extracellular infection of host structures. Furthermore, levels of secondary infection are 31 linked to conditional host fitness; B. agricolaris infected hosts have the highest level of 32 co-infection and have the highest fitness in food scarce environments. This study 33 illuminates the phenomenon of co-infection induction across Dictyostelium associated 34Burkholderia species and exemplifies the contextual complexity of these associations.

Symbiotic associations impact and are impacted by their surrounding ecosystem. The association between Burkholderia bacteria and the soil amoeba Dictyostelium discoideum is a tractable model to unravel the biology underlying symbiont-endowed phenotypes and their impacts. Several Burkholderia species stably associate with D. discoideum and typically reduce host fitness in food-rich environments while increasing fitness in food-scarce environments. Burkholderia symbionts are themselves inedible to their hosts but induce co-infections with secondary bacteria that can serve as a food source. Thus, Burkholderia hosts are “farmers” that carry food bacteria to new environments, providing a benefit when food is scarce. We examined the ability of specific Burkholderia genotypes to induce secondary co-infections and assessed host fitness under a range of co-infection conditions and environmental contexts. Although all Burkholderia symbionts intracellularly infected Dictyostelium, we found that co-infections are predominantly extracellular, suggesting that farming benefits are derived from extracellular infection of host structures. Furthermore, levels of secondary infection are linked to conditional host fitness; B. agricolaris infected hosts have the highest level of co-infection and have the highest fitness in food-scarce environments. This study illuminates the phenomenon of co-infection induction across Dictyostelium associated Burkholderia species and exemplifies the contextual complexity of these associations.

Antitumor immunity is impaired in obese mice. Mechanistic insight into this observation remains sparse and whether it is recapitulated in patients with cancer is unclear because clinical studies have produced conflicting and controversial findings. We addressed this by analyzing data from patients with a diverse array of cancer types. We found that survival after immunotherapy was not accurately predicted by body mass index or serum leptin concentrations. However, oxidized low-density lipoprotein (ox-LDL) in serum was identified as a suppressor of T-cell function and a driver of tumor cytoprotection mediated by heme oxygenase-1 (HO-1). Analysis of a human melanoma gene expression database showed a clear association between higher HMOX1 (HO-1) expression and reduced progression-free survival. Our in vivo experiments using mouse models of both melanoma and breast cancer revealed HO-1 as a mechanism of resistance to anti-PD1 immunotherapy but also exposed HO-1 as a vulnerability that could be exploited therapeutically using a small-molecule inhibitor. In conclusion, our clinical data have implicated serum ox-LDL as a mediator of therapeutic resistance in patients with cancer, operating as a double-edged sword that both suppressed T-cell immunity and simultaneously induced HO-1–mediated tumor cell protection. Our studies also highlight the therapeutic potential of targeting HO-1 during immunotherapy, encouraging further translational development of this combination approach. See article by Kuehm et al., p. 227

Checkpoint blockade immunotherapy relies on the empowerment of the immune system to fight cancer. Why some patients fail to achieve durable clinical responses is not well understood, but unique individual factors such as diet, obesity, and related metabolic syndrome could play a role. The link between obesity and patient outcomes remains controversial and has been mired by conflicting reports and limited mechanistic insight. We addressed this in a C57BL/6 mouse model of diet-induced obesity using a Western diet high in both fats and sugars. Obese mice bearing B16 melanoma or MC38 carcinoma tumors had impaired immune responses to immunotherapy and a reduced capacity to control tumor progression. Unexpectedly, these compromised therapeutic outcomes were independent of body mass and, instead, were directly attributed to dietary fructose. Melanoma tumors in mice on the high-fructose diet were resistant to immunotherapy and showed increased expression of the cytoprotective enzyme heme oxygenase-1 (HO-1). This increase in HO-1 protein was recapitulated in human A375 melanoma cells exposed to fructose in culture. Induced expression of HO-1 shielded tumor cells from immune-mediated killing and was critical for resistance to checkpoint blockade immunotherapy, which could be overcome in vivo using a small-molecule inhibitor of HO-1. This study reveals dietary fructose as a driver of tumor immune evasion, identifying HO-1 expression as a mechanism of resistance and a promising molecular target for combination cancer immunotherapy. See article by Khojandi et al., p. 214