The complicated relationship between Salmonella and the Lymph Node
Non-typhoidal Salmonella enterica (commonly referred to as Salmonella) is the leading cause of foodborne illness globally resulting in most hospitalizations and also the leading cause of foodborne illness resulting in the most deaths in the United States (CDC, 2011). In the United States, it is estimated that there are 1.3 million cases of gastroenteritis a year caused by Salmonella (Scallan et al., 2011). One in seven Salmonella outbreaks (where the contamination was found) were attributed to beef (CDC, 2008). Following the 1992-1993 Escherichia coli O157:H7 outbreak, substantial efforts have been made in the U.S. meat industry to reduce the risk of foodborne pathogens (CDC, 1993). Though most efforts targeted E. coli O157:H7, including the declaration of this pathogen as an adulterant and the establishment of zero-tolerance policy for it by the Food Safety Inspection Service (FSIS), additional efforts also reduced the presence of other foodborne pathogens. The implementation of requirements for Hazard Analysis and Critical Control Point (HACCP) systems, and other programs, following the 1993 outbreak have led to reduction in the presence of E. coli O157: H7 and the six non-O157 Shiga toxin-producing E. coli (STEC) serogroups found in beef.
Substantial work has been done investigating the source of Salmonella in beef. Multiple studies have confirmed that Salmonella can be isolated from the lymph nodes of cattle (Gragg et al., 2013b, Haneklaus, et al., 2012, Brichta-Harhay et al., 2012, Gragg et al., 2013a, Vipham et al., 2015, Moo et al., 1980). Lymph nodes in cattle, like in humans, are located all over the body. Lymph nodes can vary in size, and can be found surrounded by adipose tissue. Lymph nodes are not always distinguishable from surrounding tissue, and thus can be incorporated into ground beef (and it would be a very time consuming and near impossible task to remove over 200 lymph nodes from a beef carcass!). We know Salmonella is in bovine lymph nodes, but the big question is why?
To try to understand the complexity of the interaction of Salmonella and lymph nodes, we have to take a step back and start by familiarizing ourselves with some immunology basics. The immune system is the defensive center of the host. This system is responsible for protecting the host body from bacteria, viruses, fungi, parasites, and tumors. There are two branches of the immune system, adaptive and innate. Innate is the first line of defense, while the adaptive (or acquired) branch is specific to the pathogen or the foreign body. Physical barriers, such as skin, help to keep pathogens (also known as antigens) out of the body and stomach acid either kills or reduces the pathogens to low levels. Phagocytic cells (such as macrophages) patrol the body for pathogens and can phagocytize pathogens (basically eating pathogens). Macrophages can discriminate “self” and “foreign” molecules, like the macrophage knows the difference between a bacteria cell, and a cell from its own body. Acquired immunity comes from B and T cells that can specifically identify pathogens of interest during infection. The immune system can remember the pathogens it has encountered in the past, so that a timely immune response can be elicited in future encounters.
Lymph nodes are an incredibly important part of the immune system. Responses to pathogenic antigens are initiated and the immune response is controlled within the lymph node. The lymph node acts as a filter by grabbing antigens from the circulating lymph fluid that passes through the node (Buettner and Bode, 2012). Thus, the lymph node acts as a surveillance of the body’s tissues to identify any antigens through the flow of the lymph. Lymph nodes have three compartments and vessels that enter and exit the node. The three parts of the lymph node are the cortex, paracortex, and the medulla (Haley et al., 2005). Dendritic cells, another important cell of the immune system, are able to enter the lymph node. These Dendritic cells have an important job, they carry antigens with them to show to immune cells in the lymph node (picture a cell carrying a most wanted sign with the particular antigen, and then showing this sign to all the important cells in the lymph node so that they know there’s a criminal around). The Dendritic cells are presenting antigens to T cells in the paracortex. The medulla of the lymph node houses the macrophages. All of the lymph entering the node must pass through the medulla before exiting to the body (Gray and Cyster, 2012). Within the medulla there are macrophages (phagocytic cells) that filter and destroy particulate antigens (Willard-Mack, 2006). Macrophages internalize and degrade antigens by phagocytosis and release cytokines that alert the adaptive immune system (Gray and Cyster, 2012). This is useful if there is an active infection and bacteria are moving through the host. The macrophages in the lymph node will “catch” the bacterium, thus preventing it from causing further infection. Phagocytosis by macrophages is an important defense against pathogen invasion. Pathogens, or other material engulfed by phagocytosis by macrophages, are delivered to the phagosome, which combine with lysosomes and endosomes to enable destruction of the pathogen (Stuart and Ezekowitz, 2005; Desjardins et al., 1994). The phagolysosome of macrophages is hydrolytic and bactericidal (Garin et al., 2001; Stuart et al., 2007). The bacterium is essentially killed and broken up within the macrophage and then the macrophage presents specific parts of the antigen to other cells in the immune system.
Salmonella is a facultative intracellular pathogen. This means that it can survive outside of a host cell, and inside of a host cell. For contrast, E. coli is not an intracellular pathogen, certain strains may cause infection, but the bacteria does not enter host cells. Salmonella has specific parts of its genome that enable it to get into host cells, and survive in them. We collectively call these the Salmonella Pathogenicity Islands. Salmonella Pathogenicity Island I (SPI-I) encodes a Type 3 Secretion System (T3SS) that is essential for gastrointestinal infection which are, collectively termed the ‘invasion genes’ (Mills et al., 1995). The Pathogenicity Island II (SPI2) is required for intracellular survival. The SPI2 encodes a T3SS that is activated during intracellular conditions and is required for proliferation (Shea et al., 1996, Hensel et al.,1998). Almost immediately upon entry into the host cell a phagolysosome is formed, called the Salmonella containing vacuole (SCV) that enables intracellular growth (Mills and Finlay, 1998; Garcia-del Portillo, 2001). The SPI2 is essential for intracellular survival and dissemination throughout the lymphatic system through oral inoculation (Cirillo et al., 1998).
Now we want to go back to thinking about the immune cells in the lymph node. There are macrophages in the lymph node that are there to phagocytize pathogens. Salmonella has the ability to survive in host cells (like macrophages in the lymph nodes). Salmonella is able to put up a defense against the attacks of the macrophage, and can destroy the macrophage while continuing to infect other cells! The immune cells in the lymph node that should be killing pathogens are actually being infiltrated by Salmonella. Viable Salmonella can travel to the mesenteric lymph node by migrating dendritic cells and autonomously (Bravo-Blas, 2018). This study also observed that only half of the Salmonella in lymph are traveling inside of dendritic cells, while the rest are autonomously migrating, which is why Salmonella is recovered in higher CFUs in macrophages from the lymph node. This study, like many other studies cited here, used mouse models and mesenteric lymph nodes. Mice are vastly different then cattle, but both are vertebrates and have immune systems similar to each other. Mesenteric lymph nodes aren’t incorporated into beef muscle cuts, but the interaction of Salmonella and the immune cells documented may provide valuable information to further understand Salmonella and the bovine immune system.
Alberto Bravo-Blas, Lotta Utriainen, Slater L. Clay, Verena Kästele, Vuk Cerovic, Adam F. Cunningham, Ian R. Henderson, Daniel M. Wall, Simon W. F. Milling
The Journal of Immunology November 28, 2018, ji1701254; DOI: 10.4049/jimmunol.1701254
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