Aim: Immunostimulation, before the fish is able to mount an effective immune response, can induce tolerance. Therefore, present study was carried out to investigate the structural development of lymphoid organs, namely thymus, kidney and spleen of Indian major carp, Catla (Catla catla), which will help in deciding the appropriate age, when immunocompetence is established. Methodology: In this study, ontogeny of the lymphoid organs of catla was studied from hatching up to 54 days post hatch (dph). For this, whole fish were fixed, processed and embedded in paraffin wax. Further, serial sections were cut, stained and observed under microscope. Results: The thymus anlage appeared in the dorso-anterior part of the gill cavity at 1 dph. At 5 dph, an undifferentiated mass of stem cells was observed and by 8 dph, it appeared lymphoid. At 11 dph, clear dark and light zones of cells were observed. At 26 dph, Hassall's corpuscles were observed and no further changes were observed except increase in number of lymphoid cells till end of the sampling period. Kidney was observed as an undifferentiated mass of stem cells at 1 dph with few renal tubules. At 5 dph, undifferentiated stem cells increased in number, and by 8 dph, few erythropoietic cells were observed. At 11 dph, lymphoid cells were present and by 17 dph, renal tubules started degenerating. At 26 dph, anterior kidney completely lacked excretory tissue and appeared lymphoid. Spleen was observed at 3 dph as an undifferentiated mass of stem cells, and at 8 dph, erythrocytes were seen. At 11 dph, spleen was heavily packed with erythrocytes, whereas at 20 dph, the number of erythrocytes decreased and lymphocytes were observed. Fully developed spleen was observed at 33 dph with presence of red and white pulp. Interpretation: Thymus, head kidney and spleen were fully developed on 33 dph. The information generated in the present study would be helpful in deciding the appropriate age for vaccination

India is the second largest fish producing country in the world with production of 12.6 million tonnes (mt) in 2017–18, and Indian major carps (IMCs) contribute bulk of this fish production. Catla, Catla catla is the fastest growing species among IMCs. However, the survival rate of catla during larval rearing is normally lower than the other IMCs i.e rohu Labeo rohita, and mrigal Cirrhinus mrigala. Continuous efforts are devoted for the identification of nutritional and environmental requirements of fish larvae in order to reduce hatchery mortalities. However, very little information is available regarding physiology of the immune system, especially during the late larval and juvenile stages. Hence, understanding the ontogenetic development of immune-relevant genes in the larval stages of catla will serve as the markers for the development of immune competence and thereby, will be beneficial in developing effective immune intervention strategies. In the present study, expression profiles of some of the important innate (IL-1β, IL-10, iNOS and C3) and adaptive immune (RAG-1, Ikaros, IgM and IgZ) genes during ontogenetic developmental stages and in different tissues of catla were investigated using quantitative real-time PCR. The results revealed that immune genes IL-1β, C3, IgM and IgZ were detected in the unfertilized eggs indicating their maternal inheritance. Immune genes, IL-1β, IL-10 and iNOS were expressed significantly during initial larval developmental stages whereas C3, RAG-1, Ikaros, IgM and IgZ showed significant expression during advanced stages of larval development in catla i.e. from 23 days post hatch (dph). Study of tissue distribution pattern of the genes indicated that innate immune genes were ubiquitously expressed in different tissues with varying degree of expression, whereas adaptive immune genes were predominantly expressed in lymphoid organs of catla. The information thus generated will improve knowledge on the maturation of the immune system in catla and will aid in deciding the appropriate age for vaccination in this teleost species.

Columnaris disease, caused by Flavobacterium columnare, is one of the important bacterial diseases responsible for large-scale mortalities in numerous freshwater fishes globally. This disease can cause up to 100% mortality within 24 h of infection and is considered to be a cause of concern for aquaculture industry. Despite being a serious disease, scarce information is available regarding host-pathogen interaction, particularly the modulation of different immune genes in response to F. columnare infection. Therefore, in the present study, an attempt has been made to study expression of important immune regulatory genes, namely IL-1β, iNOS, INF-γ, IL-10, TGF-β, C3, MHC-I and MHC-II in gills and kidney of Catla catla following experimental infection with F. columnare. The expression analysis of immune genes revealed that transcript levels of IL-1β, iNOS, IL-10, TGF-β, C3 and MHC-I were significantly up-regulated (p < 0.05) in both the organs of the infected catla. IFN-γ and MHC-II were up-regulated in gills of infected catla whereas, both the genes showed down-regulation in kidney. The results indicate that important immune genes of C. catla are modulated following infection with F. columnare. The knowledge thus generated will strengthen the understanding of molecular pathogenesis of F. columnare in Indian major carp C. catla.