Which cells are primarily responsible for triggering apoptosis in macrophages?

The primary role of macrophages in the immune response is to phagocytose pathogens and trigger various inflammatory responses. One mechanism through which macrophages can be modulated in their activity is through apoptosis, which can be triggered by certain pathogens.

In the context of the options presented, Shigella is known to exploit the host cellular machinery to facilitate its invasion and, in particular, can induce apoptosis in immune cells, including macrophages. This is a tactic that some pathogens use to evade the immune system, allowing them to survive and replicate within the host. Shigella's ability to manipulate the apoptotic pathways of macrophages not only aids in its own survival but can also help in the spread of infection by subverting the typical immune response.

The other pathogens listed—Escherichia coli, Staphylococcus aureus, and Streptococcus pneumoniae—have different interactions with macrophages and do not primarily induce apoptosis in the same way that Shigella does. While they may have various mechanisms of immune evasion and modulation, they do not specifically trigger apoptosis in macrophages as a defining characteristic of their pathogenicity. Thus, Shigella is correctly identified as the key pathogen responsible for inducing apoptosis in these immune cells.