This is a Phase 1/2 open label trial of G100 in patients with low grade NHL. G100 is composed of glucopranosyl lipid A in a stable emulsion and is a potent TLR4 (toll-like receptor-4) agonist. G100 will be administered by direct injection (intratumorally) into tumors of low grade NHL following standard low dose radiation therapy. Preclinical models and clinical studies in other cancers such as Merkel cell carcinoma have demonstrated that G100 administered in this manner can alter the tumor microenvironment, activate dendritic cells, T cells and other immune cells and induce systemic anti-tumor immune responses. In this trial, the safety, immunogenicity, and clinical efficacy of G100 will be examined alone or with pembrolizumab.