An electron microscope study of lymphatic tissue in runt disease

The PD-1 expression balance between effector and regulatory T cells predicts the clinical efficacy of PD-1 blockade therapies

Immune checkpoint blockade has provided a paradigm shift in cancer therapy, but the success of this approach is very variable; therefore, biomarkers predictive of clinical efficacy are urgently required. Here, we show that the frequency of PD-1⁺CD8⁺ T cells relative to that of PD-1⁺ regulatory T (T_reg) cells in the tumor microenvironment can predict the clinical efficacy of programmed cell death protein 1 (PD-1) blockade therapies and is superior to other predictors, including PD ligand 1 (PD-L1) expression or tumor mutational burden. PD-1 expression by CD8⁺ T cells and T_reg cells negatively impacts effector and immunosuppressive functions, respectively. PD-1 blockade induces both recovery of dysfunctional PD-1⁺CD8⁺ T cells and enhanced PD-1⁺ T_reg cell-mediated immunosuppression. A profound reactivation of effector PD-1⁺CD8⁺ T cells rather than PD-1⁺ T_reg cells by PD-1 blockade is necessary for tumor regression. These findings provide a promising predictive biomarker for PD-1 blockade therapies.

Thymic involution in murine graft-versus-host reaction. Epithelial injury mimicking human thymic dysplasia

Mild, moderate, and severe graft-versus-host (GVH) reactions were induced in four series of experiments in 71 CBA X A and C57BL/6 X A F1 hybrid mice. At regular intervals post-GVH reaction induction (Days 4-42), the animals were sacrificed, autopsied, and histologically studied. Visceral alterations of GVH reaction were recorded in the spleen, lymph nodes, liver, kidney, gut, and thymus. A spectrum of thymic changes was documented, ranging from obliteration of a definable cortex and medulla with loss of Hassall's corpuscles to marked involution with complete disappearance of the gland. Ultrastructural studies revealed damage to both lymphocytes and epithelial cells along with lymphocyte emperipolesis of epithelial cells, lymphocytolysis within epithelial cells, and accumulation of numerous autophagic vacuoles containing fragments of cellular debris within epithelial cells and histiocytes. The resemblance of these alterations to human thymic dysplasia as observed in primary immunodeficient conditions was striking. The theoretical implications of these studies for the pathogenesis of human congenital immunodeficiency states are considered.

Antigens in immunity. XIV. Electron microscopic radioautographic studies of antigen capture in the lymph node medulla

Details of antigen trapping and processing in the rat lymph node have been investigated by the technique of high resolution radioautography. A series of 24 adult rats was injected with 20 microg of (125)I-labeled Salmonella adelaide flagella, given as either a primary or a secondary stimulus into one hind foot-pad. At intervals ranging from 3 min to 3 wk, rats were killed and the popliteal nodes were processed for electron microscopic radioautography using Kodak NTE emulsion. The present paper deals with events in the lymph node medulla, and an accompanying report describes the radically different behavior of antigen in the cortical follicles. In the medulla, lightly labeled granulocytes were transiently encountered, but by far the greatest bulk of antigen was in macrophages. Antigen entered these cells in two ways: by direct penetration of the plasma membrane; and by pinocytosis. In either case, the antigen rapidly became surrounded by tiny vesicles which may have represented Golgi-derived "protolysosomes." Vacuolar fusion ensued and a series of progressively larger and more complex antigen-containing "phagolysosomes" was formed. Substantial amounts of antigen could be detected in such bodies for at least 3 wk. The antigen injection, as expected, caused extensive plasma-cytopoiesis. No evidence of label in plasma cells was obtained. No special anatomic relationship between plasma cells and antigen depot sites was discovered. These results are briefly discussed in relation to current theories of immune induction.