Figure 2.

The immune mechanisms of BCG delivered by mucosal and intravenous vaccination. (a). BCG is first taken up by M cells of mucosal epithelium and transported to mucosa-associated lymphoid tissue (MALT). After BCG is processed by dendritic cells, effector T and B lymphocytes are generated, and then differentiated into memory cells. The effector T and B lymphocytes play their protective functions in the effective sites after lymphatic circulation and blood circulation. Except for that tissue-resident memory T (TRM) cells remain constrained within local tissue, central memory T (TCM) cells and effector memory T (TEM) cells migrate to the corresponding lymphatic organs or non-lymphoid tissues. When the body is attacked by Mtb, TRM cells respond quickly, and then the circulating memory cells perform their effector functions. At the same time, memory B cells also rapidly differentiate and secrete IgA (sIgA). (b). Darrah groups¹⁶ showed IV BCG made that 9 out of 10 macaques were highly protected and even 6 showed no signs of infection. The possible protective mechanism of IV BCG vaccination: increased markedly antigen-responsive T cells, higher significantly antibody response, and well-trained immunity. Red shows the presence of bacteria and pulmonary tuberculosis disease, and Orange indicates reduced bacterial burdens and disease, whereas brown remarks no detectable infection.